<PAGE>   1

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                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                            ------------------------

                                   FORM 10-Q

[X]   QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
      EXCHANGE ACT OF 1934

     FOR THE QUARTERLY PERIOD ENDED MARCH 31, 2000

     OR

[ ]   TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
      EXCHANGE ACT OF 1934

           FOR THE TRANSITION PERIOD FROM             TO

                       COMMISSION FILE NUMBER: 000-24931

                                 S1 CORPORATION
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

<TABLE>
<S>                                             <C>
                  DELAWARE                                       58-2395199
      (STATE OR OTHER JURISDICTION OF                         (I.R.S. EMPLOYER
       INCORPORATION OR ORGANIZATION)                       IDENTIFICATION NO.)

    3390 PEACHTREE ROAD, NE, SUITE 1700                            30326
              ATLANTA, GEORGIA                                   (ZIP CODE)
  (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)
</TABLE>

       REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE: (404) 812-6200

                                 NOT APPLICABLE
                  (FORMER NAME IF CHANGED SINCE LAST REPORT.)

     Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports) and (2) has been subject to such
filing requirements for the past 90 days. Yes [X]  No [ ].

     Shares of common stock outstanding as of May 12, 2000: 54,190,407

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<PAGE>   2

                        S1 CORPORATION AND SUBSIDIARIES

                          CONSOLIDATED BALANCE SHEETS
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                               MARCH 31,     DECEMBER 31,
                                                                 2000            1999
                                                              -----------    -------------
                                                              (IN THOUSANDS, EXCEPT SHARE
                                                                  AND PER SHARE DATA)
<S>                                                           <C>            <C>
                                          ASSETS
Current assets:
  Cash and cash equivalents.................................  $   75,109      $   67,850
  Investment securities available for sale (cost of $1,343
    at March 31, 2000 and $1,800 at December 31, 1999)......      38,549          62,754
  Accounts receivable, net of allowance for doubtful
    accounts and billing adjustments of $8,036 at March 31,
    2000 and $8,584 at December 31, 1999....................      63,662          70,136
  Notes receivable..........................................       1,500           1,500
  Prepaid expenses..........................................       7,908           6,625
  Other current assets......................................         624             678
                                                              ----------      ----------
         Total current assets...............................     187,352         209,543
  Property and equipment, net...............................      33,553          24,580
  Intangible assets, net....................................      94,621         106,508
  Notes receivable..........................................       2,004              --
  Goodwill, net.............................................     719,615         788,293
  Other assets..............................................       6,372           3,563
                                                              ----------      ----------
         Total assets.......................................  $1,043,517      $1,132,487
                                                              ==========      ==========

                           LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
  Accounts payable..........................................  $   18,443      $   29,493
  Accrued salaries and benefits.............................      11,242          12,123
  Accrued other expenses....................................      56,225          44,122
  Deferred revenues.........................................      22,218          20,469
  Notes payable.............................................       3,744           6,351
  Current portion of capital lease obligation...............         615             759
                                                              ----------      ----------
         Total current liabilities..........................     112,487         113,317
  Deferred revenues.........................................       5,426           9,283
  Capital lease obligation, excluding current portion.......         916           1,086
  Deferred tax liability....................................      15,386          15,386
  Other liabilities.........................................       2,125           2,608
                                                              ----------      ----------
         Total liabilities..................................     136,340         141,680
                                                              ----------      ----------
  Stockholders' equity:
    Preferred stock, $0.01 par value. Authorized 25,000,000
     shares. Issued and outstanding 1,061,514 and 1,393,014
     shares at March 31, 2000 and December 31, 1999,
     respectively...........................................      22,675          23,089
    Common stock, $0.01 par value. Authorized 350,000,000
     shares. Issued and outstanding 51,163,353 and
     48,831,243 shares at March 31, 2000 and December 31,
     1999, respectively.....................................         512             488
    Additional paid-in capital..............................   1,142,127       1,126,607
    Receivable from the sale of stock.......................     (11,730)        (11,735)
    Accumulated deficit.....................................    (283,135)       (207,927)
    Accumulated other comprehensive income:
      Net unrealized gains on investment securities
       available for sale, net of taxes.....................      36,394          60,143
      Cumulative foreign currency translation adjustment,
       net of taxes.........................................         334             142
                                                              ----------      ----------
         Total stockholders' equity.........................     907,177         990,807
                                                              ----------      ----------
         Total liabilities and stockholders' equity.........  $1,043,517      $1,132,487
                                                              ==========      ==========
</TABLE>

     See accompanying notes to unaudited consolidated financial statements.
                                        2
<PAGE>   3

                        S1 CORPORATION AND SUBSIDIARIES

                     CONSOLIDATED STATEMENTS OF OPERATIONS
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                                 THREE MONTHS ENDED
                                                                     MARCH 31,
                                                              ------------------------
                                                                 2000          1999
                                                              ----------    ----------
                                                               (IN THOUSANDS, EXCEPT
                                                                SHARE AND PER SHARE
                                                                       DATA)
<S>                                                           <C>           <C>
Revenues:
  Software licenses.........................................  $   10,719    $    2,308
  Professional services.....................................      34,382         7,722
  Data center...............................................       3,507         1,547
  Other.....................................................       1,761           423
                                                              ----------    ----------
          Total revenues....................................      50,369        12,000
                                                              ----------    ----------
Direct costs:
  Software licenses.........................................       1,521           133
  Professional services, excluding stock compensation
     expense of $24 in 2000 and 1999, respectively..........      25,884         4,955
  Data center...............................................       3,352         1,687
  Other.....................................................       1,594           344
                                                              ----------    ----------
          Total direct costs................................      32,351         7,119
                                                              ----------    ----------
          Gross margin......................................      18,018         4,881
Operating expenses:
  Selling and marketing, excluding stock compensation
     expense of $188 in 2000................................      11,406         1,079
  Product development, excluding stock compensation expense
     of $440 and $54 in 2000 and 1999, respectively.........      14,992         4,321
  General and administrative, excluding stock compensation
     expense of $473 and $29 in 2000 and 1999,
     respectively...........................................       9,351         1,562
  Depreciation and amortization.............................       3,404         1,194
  Stock option compensation expense.........................       1,125           107
  Marketing cost from warrant issued........................       4,600            --
  Merger related costs......................................       6,814            --
  Amortization of acquisition intangibles...................      77,127           103
                                                              ----------    ----------
          Total operating expenses..........................     128,819         8,366
                                                              ----------    ----------
          Operating loss....................................    (110,801)       (3,485)
Interest and investment income..............................      35,593           227
                                                              ----------    ----------
Net loss....................................................  $  (75,208)   $   (3,258)
                                                              ==========    ==========
Basic and diluted net loss per common share.................  $    (1.49)   $    (0.13)
                                                              ==========    ==========
Weighted average number of shares of common stock
  outstanding...............................................  50,456,210    24,698,334
                                                              ==========    ==========
</TABLE>

     See accompanying notes to unaudited consolidated financial statements.
                                        3
<PAGE>   4

                        S1 CORPORATION AND SUBSIDIARIES

 CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY AND COMPREHENSIVE INCOME (LOSS)
                   FOR THE THREE MONTHS ENDED MARCH 31, 2000
                                  (UNAUDITED)
<TABLE>
<CAPTION>
                                                    CONVERTIBLE PREFERRED STOCK
                                     ----------------------------------------------------------
                                         SERIES A            SERIES B             SERIES C           COMMON STOCK       ADDITIONAL
                                     -----------------   -----------------   ------------------   -------------------    PAID-IN
                                      SHARES    AMOUNT   SHARES    AMOUNT     SHARES    AMOUNT      SHARES     AMOUNT    CAPITAL
                                     --------   ------   -------   -------   --------   -------   ----------   ------   ----------
                                                                   (IN THOUSANDS, EXCEPT SHARE DATA)
<S>                                  <C>        <C>      <C>       <C>       <C>        <C>       <C>          <C>      <C>
Balance December 31, 1999..........   428,950   $1,062   749,064   $10,000    215,000   $12,027   48,831,243    $488    $1,126,607
Net loss...........................        --      --         --       --          --       --            --      --            --
Change in net unrealized gains on
 investment securities available
 for sale, net of taxes............        --      --         --       --          --       --            --      --            --
Change in cumulative foreign
 currency translation adjustment,
 net of taxes......................        --      --         --       --          --       --            --      --            --
Realized gain on sale of investment
 securities available for sale.....        --      --         --       --          --       --            --      --            --
Conversion of preferred stock to
 common stock......................  (331,500)   (414)        --       --          --       --       663,000       7           407
Payment on receivable from the sale
 of stock..........................        --      --         --       --          --       --            --      --            --
Interest earned on receivable from
 the sale of stock.................        --      --         --       --          --       --            --      --           235
Common stock sold under employee
 stock purchase and option plans...        --      --         --       --          --       --     1,669,110      17         9,153
Deferred stock option compensation
 expense...........................        --      --         --       --          --       --            --      --         1,125
Warrant issued in connection with
 marketing agreement...............        --      --         --       --          --       --            --      --         4,600
Comprehensive loss.................
                                     --------   ------   -------   -------   --------   -------   ----------    ----    ----------
Balance March 31, 2000.............    97,450   $ 648    749,064   $10,000    215,000   $12,027   51,163,353    $512    $1,142,127
                                     ========   ======   =======   =======   ========   =======   ==========    ====    ==========

<CAPTION>

                                     RECEIVABLE                  ACCUMULATED
                                      FROM THE                      OTHER           TOTAL
                                      SALE OF     ACCUMULATED   COMPREHENSIVE   STOCKHOLDERS'   COMPREHENSIVE
                                       STOCK        DEFICIT        INCOME          EQUITY       INCOME (LOSS)
                                     ----------   -----------   -------------   -------------   -------------
                                                        (IN THOUSANDS, EXCEPT SHARE DATA)
<S>                                  <C>          <C>           <C>             <C>             <C>
Balance December 31, 1999..........   $(11,735)    $(207,927)     $ 60,285        $990,807
Net loss...........................         --       (75,208)           --         (75,208)       $(75,208)
Change in net unrealized gains on
 investment securities available
 for sale, net of taxes............         --            --          (243)           (243)           (243)
Change in cumulative foreign
 currency translation adjustment,
 net of taxes......................         --            --           192             192             192
Realized gain on sale of investment
 securities available for sale.....         --            --       (23,506)        (23,506)             --
Conversion of preferred stock to
 common stock......................         --            --            --              --              --
Payment on receivable from the sale
 of stock..........................        240            --            --             240              --
Interest earned on receivable from
 the sale of stock.................       (235)           --            --              --              --
Common stock sold under employee
 stock purchase and option plans...         --            --            --           9,170              --
Deferred stock option compensation
 expense...........................         --            --            --           1,125              --
Warrant issued in connection with
 marketing agreement...............         --            --            --           4,600              --
Comprehensive loss.................                                                               $(75,259)
                                      --------     ---------      --------        --------        ========
Balance March 31, 2000.............   $(11,730)    $(283,135)     $ 36,728        $907,177
                                      ========     =========      ========        ========
</TABLE>

     See accompanying notes to unaudited consolidated financial statements.

                                        4
<PAGE>   5

                        S1 CORPORATION AND SUBSIDIARIES

                     CONSOLIDATED STATEMENTS OF CASH FLOWS
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                              THREE MONTHS ENDED
                                                                   MARCH 31,
                                                              -------------------
                                                                2000       1999
                                                              --------    -------
                                                                (IN THOUSANDS)
<S>                                                           <C>         <C>
Cash flows from operating activities:
  Net loss..................................................  $(75,208)   $(3,258)
  Adjustments to reconcile net loss to net cash (used in)
     provided by operating activities:
       Depreciation and amortization........................    80,531      1,297
       Compensation and marketing expense for stock options
        and warrant.........................................     5,725        107
       Provision for doubtful accounts receivable and
        billing adjustments.................................       923        490
       Gain on the sale of investment securities available
        for sale............................................   (34,818)        --
       Decrease in accounts receivable......................     5,551      9,437
       Increase in prepaid expenses and other assets........    (1,496)      (704)
       Decrease in accounts payable.........................   (11,050)    (2,335)
       Increase in accrued expenses and other liabilities...    10,560         59
       (Decrease) increase in deferred revenue..............    (2,108)     2,492
                                                              --------    -------
          Net cash (used in) provided by operating
           activities.......................................   (21,390)     7,585
                                                              --------    -------
Cash flows from investing activities:
  Proceeds from sales of investment securities available for
     sale...................................................    36,546         --
  Investments in and advances to unconsolidated companies...    (5,538)        --
  Purchases of property and equipment and purchased
     technology.............................................    (8,933)      (741)
                                                              --------    -------
          Net cash provided by (used in) investing
           activities.......................................    22,075       (741)
                                                              --------    -------
Cash flows from financing activities:
  Payment on subscription receivable........................       240        962
  Proceeds from sale of common stock under employee stock
     purchase and option plans..............................     9,170        922
  Payments on capital lease obligations.....................      (315)      (514)
  Payments on borrowings....................................    (2,432)        --
  Proceeds from interest income on receivable from sale of
     stock..................................................        --         80
                                                              --------    -------
          Net cash provided by financing activities.........     6,663      1,450
                                                              --------    -------
Effect of exchange rate changes on cash.....................       (89)         5
                                                              --------    -------
Net increase in cash........................................     7,259      8,299
Cash and cash equivalents at beginning of period............    67,850     14,504
                                                              --------    -------
Cash and cash equivalents at end of period..................  $ 75,109    $22,803
                                                              ========    =======
Noncash financing activities:
  Conversion of preferred stock to common stock.............       414        798
  Issuance of Series C Preferred Stock in exchange for
     subscription receivable................................        --     12,027
</TABLE>

     See accompanying notes to unaudited consolidated financial statements.
                                        5
<PAGE>   6

                        S1 CORPORATION AND SUBSIDIARIES

                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                                  (UNAUDITED)

1. BASIS OF PRESENTATION

     S1 Corporation (the "Company" or "S1") develops integrated Internet
software applications that enable financial organizations to offer products,
services and transactions over the Internet in a secure environment. S1
Corporation also offers product integration, training, call center technology
and data center processing services.

     The consolidated financial statements include the accounts of S1 and its
wholly owned subsidiaries. Significant intercompany accounts and transactions
have been eliminated in consolidation. The consolidated financial statements for
the three month periods ended March 31, 2000 and 1999 are unaudited and do not
include information or footnotes necessary for a complete presentation of
financial condition, results of operations and cash flows. The interim financial
statements include all adjustments, consisting only of normal recurring
adjustments, which in the opinion of management are necessary to present fairly
the Company's consolidated financial statements. The results of operations for
the three months ended March 31, 2000 are not necessarily indicative of the
expected results for the year ending December 31, 2000.

2. PENDING BUSINESS COMBINATIONS

     On April 7, 2000, the Company completed its acquisition of Q Up Systems,
Inc. ("Q Up"), a leading provider of Internet banking and e-commerce portal
solutions for financial institutions, primarily community banks. Under the
agreement, the Company issued approximately 2.4 million shares of common stock
to the shareholders of Q UP in exchange for the shares of the company, including
preferred shares. In addition, the Company issued approximately 1.4 million
options in exchange for options held by existing Q Up employees. The transaction
will be accounted for using the purchase method of accounting.

     On April 13, 2000, the Company completed its acquisition of Davidge Data
Systems ("Davidge"), a software company that specializes in middleware and order
routing software and systems for the brokerage industry. Under the agreement,
the Company acquired Davidge for approximately 250,000 shares of common stock.
In addition, the Company will issue approximately 496,000 options both in
exchange for options held by existing Davidge employees and as additional
options to Davidge employees. The transaction will be accounted for using the
purchase method of accounting.

3. COMMITMENTS AND CONTINGENCIES

     On March 1, 2000, the Company entered into a ten year operating lease for a
total of 185,000 square feet of office space, commencing September 2001. The
minimum annual payments under the lease are $1.4 million in 2001, $4.2 million
in 2002, $4.3 million in 2003, $4.4 million in 2004, and $32.2 million in years
thereafter.

     In May 2000, the Company and three of its executive officers were named in
three shareholder lawsuits (see Part II, Item 1 of this Form 10-Q). While the
ultimate results and outcomes of these actions and claims cannot be determined,
management does not expect that they will have a material adverse effect on the
Company's results of operations or financial position.

4. NET LOSS PER SHARE

     Basic net loss per share is calculated as loss available to common
stockholders divided by the weighted average number of common shares outstanding
during the period. Diluted earnings per share is calculated to reflect the
potential dilution that would occur if stock options or other contracts to issue
common stock were exercised and resulted in additional common stock that would
share in the earnings of the Company. Because of the Company's net losses, the
issuance of additional shares of common stock under stock options and warrants
or upon the conversion of preferred stock would be antidilutive. The total
number of common shares

                                        6
<PAGE>   7
                        S1 CORPORATION AND SUBSIDIARIES

           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                                  (UNAUDITED)

that would have been used in the Company's computation of diluted earnings per
share for the three month period ended March 31, 2000 and 1999 was 66,757,024
and 36,407,070 respectively.

5. SEGMENT REPORTING

     S1 operates in three business segments, Internet Financial Services, Call
Center Technology and Internet Aggregation Services. The Internet Financial
Services segment builds, delivers and operates integrated, transactional and
brandable Internet applications for financial institutions worldwide, available
as an in-house solution or outsourced to the S1 Data Center. Through the Call
Center Technology segment, S1 offers interactive voice response and voice
e-Commerce technology. The Internet Aggregation Services segment provides
technology that enables consumers to aggregate personal account information from
multiple sources. S1 manages the business based on these operating segments.

     S1 evaluates the performance of its operating segments based on revenues,
direct costs and operating income, excluding depreciation and amortization,
stock option compensation expense, marketing cost from a warrant issued, merger
related costs and amortization of acquisition intangibles. In addition, the
Company provides general and administrative services to the operating segments
on a shared services basis and does not allocate these costs to the individual
segments, therefore the general and administrative costs are included in the
other category. The Company does not produce reports that measure the
performance according to any asset-based metrics.

     For the three months ended March 31, 2000, one major customer accounted for
approximately 36% of total revenue. For the three months ended March 31, 1999,
three major customers accounted for approximately 44%, 17% and 12% of total
revenue, respectively.

<TABLE>
<CAPTION>
                                                       QUARTER ENDED MARCH 31, 2000
                                      --------------------------------------------------------------
                                      INTERNET      INTERNET         CALL
                                      FINANCIAL    AGGREGATION      CENTER
                                      SERVICES      SERVICES      TECHNOLOGY     OTHER       TOTAL
                                      ---------    -----------    ----------    -------    ---------
                                                          (DOLLARS IN THOUSANDS)
                                                               (UNAUDITED)
<S>                                   <C>          <C>            <C>           <C>        <C>
Revenues:
  Software licenses.................   $ 3,666       $    --       $ 7,053      $    --    $  10,719
  Professional services.............    30,969            --         3,413           --       34,382
  Data center.......................     3,193           314            --           --        3,507
  Other.............................     1,761            --            --           --        1,761
                                       -------       -------       -------      -------    ---------
          Total revenues............    39,589           314        10,466           --       50,369
                                       -------       -------       -------      -------    ---------
Direct costs:
  Software licenses.................        24            --         1,497           --        1,521
  Professional services.............    23,351            --         2,533           --       25,884
  Data center.......................     3,154           198            --           --        3,352
  Other.............................     1,594            --            --           --        1,594
                                       -------       -------       -------      -------    ---------
          Total direct costs........    28,123           198         4,030           --       32,351
                                       -------       -------       -------      -------    ---------
          Gross margin..............    11,466           116         6,436           --       18,018
Operating expenses..................    18,134         4,498         7,699        5,418       35,749
                                       -------       -------       -------      -------    ---------
          Operating loss*...........   $(6,668)      $(4,382)      $(1,263)     $(5,418)     (17,731)
                                       =======       =======       =======      =======
</TABLE>

                                        7
<PAGE>   8
                        S1 CORPORATION AND SUBSIDIARIES

           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                       QUARTER ENDED MARCH 31, 2000
                                      --------------------------------------------------------------
                                      INTERNET      INTERNET         CALL
                                      FINANCIAL    AGGREGATION      CENTER
                                      SERVICES      SERVICES      TECHNOLOGY     OTHER       TOTAL
                                      ---------    -----------    ----------    -------    ---------
                                                          (DOLLARS IN THOUSANDS)
                                                               (UNAUDITED)
<S>                                   <C>          <C>            <C>           <C>        <C>
  Depreciation and amortization.....                                                           3,404
  Stock option compensation
     expense........................                                                           1,125
  Marketing cost from warrant
     issued.........................                                                           4,600
  Merger related costs..............                                                           6,814
  Amortization of acquisition
     intangibles....................                                                          77,127
                                                                                           ---------
Operating loss in the income
  statement.........................                                                       $(110,801)
                                                                                           =========
</TABLE>

---------------
* Excluding depreciation and amortization, stock option compensation expense,
  marketing cost from warrant issued, merger related costs and amortization of
  acquisition intangibles.

<TABLE>
<CAPTION>
                                                          QUARTER ENDED MARCH 31, 1999
                                           ----------------------------------------------------------
                                           INTERNET      INTERNET         CALL
                                           FINANCIAL    AGGREGATION      CENTER
                                           SERVICES      SERVICES      TECHNOLOGY    OTHER     TOTAL
                                           ---------    -----------    ----------    -----    -------
<S>                                        <C>          <C>            <C>           <C>      <C>
Revenues:
  Software licenses......................   $ 2,308         $--            $--        $--     $ 2,308
  Professional services..................     7,722         --             --         --        7,722
  Data center............................     1,547         --             --         --        1,547
  Other..................................       423         --             --         --          423
                                            -------         --             --         --      -------
          Total revenues.................    12,000         --             --         --       12,000
                                            -------         --             --         --      -------
Direct costs:
  Software licenses......................       133         --             --         --          133
  Professional services..................     4,955         --             --         --        4,955
  Data center............................     1,687         --             --         --        1,687
  Other..................................       344         --             --         --          344
                                            -------         --             --         --      -------
          Total direct costs.............     7,119         --             --         --        7,119
                                            -------         --             --         --      -------
          Gross margin...................     4,881         --             --         --        4,881
Operating expenses.......................     6,962         --             --         --        6,962
                                            -------         --             --         --      -------
          Operating loss*................   $(2,081)        $--            $--        $--      (2,081)
                                            =======         ==             ==         ==
  Depreciation and amortization..........                                                       1,194
  Stock option compensation expense......                                                         107
  Amortization of acquisition
     intangibles.........................                                                         103
                                                                                              -------
Operating loss in the income statement...                                                     $(3,485)
                                                                                              =======
</TABLE>

---------------
* Excluding depreciation and amortization, stock option compensation expenses
  and amortization of acquisition intangibles.

                                        8
<PAGE>   9

ITEM 2.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
       OF OPERATIONS.

     The statements contained in this quarterly report that are forward-looking
are based on current expectations that are subject to a number of uncertainties
and risks, and actual results may differ materially. These forward-looking
statements are not guarantees of future performance and are subject to risks and
uncertainties that could cause actual results to differ materially from the
results contemplated by the forward-looking statements. These risks and
uncertainties include, but are in no way limited to:

     - the possibility that the anticipated benefits from our recent acquisition
       transactions will not be fully realized;

     - the possibility that costs or difficulties related to our integration of
       recent acquisitions will be greater than expected;

     - our dependence on the timely development, introduction and customs
       acceptance of new internet services;

     - rapidly changing technology and shifting demand requirements and internet
       usage patterns;

     - other risks and uncertainties, including the impact of competitive
       services, products and prices, the unsettled conditions in the internet
       and other high-technology industries and the ability to attract and
       retain key personnel; and

     - other risk factors as may be detailed from time to time in our public
       announcements and filings with the SEC, including the Company's annual
       report on Form 10-K for the year ended December 31, 1999.

     In addition, nothing in this quarterly report should be viewed as an update
or comment on earlier forward looking statements provided by S1 Corporation. As
noted above, because actual results, performance or developments may differ
materially from forward-looking statements, S1 will not update such statements
over the course of future periods.

     The following discussion should be read in conjunction with the unaudited
consolidated financial statements and notes appearing elsewhere herein and in
our Form 10-K for the year ended December 31, 1999.

GENERAL

     We are a leading global infrastructure provider of Internet-based solutions
for financial organizations. Through our "engage, enable, everywhere" strategy,
we provide the solutions that empower our client banks, brokerage firms and
insurance companies to engage and enable their customers to access their
financial information and conduct transactions everywhere the Internet is
available. We believe our solutions empower our clients to increase revenue,
strengthen customer relationships and gain competitive advantage by meeting the
evolving needs of their customers across various lines of business, market
segments and delivery channels.

     We provide a comprehensive set of Internet-based financial services
solutions which touch every significant facet of the financial services
industry. From banking, brokerage and insurance to personal financial management
and tax preparation capabilities, our suites of consumer, retail, small business
and corporate applications provide a comprehensive set of solutions for global,
national, regional and local financial organizations. Our solutions, including
our VerticalOne personal information aggregation service and our Edify
interactive voice response technology, reach beyond traditional online financial
services to help financial organizations deliver value to their customers,
resulting in greater customer retention and increased revenue opportunities for
our clients. In addition, we have implemented wireless solutions in the United
States, Europe and Australia that allow our clients' customers to access account
information and conduct transactions through cellular telephones and hand-held
personal digital assistants.

     On November 10, 1999, we completed the acquisitions of Edify and
VerticalOne. On November 18, 1999, we completed the acquisition of FICS. All
three acquisitions have been accounted for as purchase business combinations.
Accordingly, the results of operations for the three months ended March 31, 1999
do not include the results of operations for these acquired companies.
                                        9
<PAGE>   10

  Revenues

     We derive our revenues primarily from three sources:

     Software licenses.  We receive license fees from direct licensees and
third-party data processors. Direct licensees install and operate our products
in their own environments. We generally receive an initial license fee plus
ongoing fees which are based on either the number of end-users or a percentage
of the initial license fee. For software license sales where no significant
obligations remain, revenue is recognized upon delivery of the software,
provided collection is considered probable, the fee is fixed or determinable,
there is evidence of an arrangement, and vendor specific evidence exists to
allocate the total fee to all elements of the arrangement. When services are
considered essential to the functionality of the software, the software license
and the related services are recognized over the implementation period using the
percentage of completion method of accounting.

     A portion of our software license revenue is being recognized on a
straight-line basis over either the term of the agreement or, for contracts
without a term, the estimated period during which post-contract support is
expected to be provided. Under these arrangements, post-contract support and
maintenance were bundled as part of the license agreements and sufficient vendor
specific evidence did not exist to allocate the total fee to all elements of the
arrangement.

     Third-party data processors install our products in their own data
processing centers and license the product to their client institutions,
typically smaller financial services entities like community banks and thrifts.
We receive monthly fees from third party data processors based on the total
number of end-users served by the processors' client institutions. These fees
are recognized as revenue in the period earned.

     Professional Services.  We provide professional services related to the
installation and integration of our products. These services include:

     - installing the product at direct licensees and third-party data
       processing centers;

     - integrating the financial organization data processing systems with our
       data center for data center clients;

     - providing product enhancements;

     - consulting; and

     - training.

     Revenues derived from contracts to provide services on a time and materials
basis are recognized as the related services are performed. Revenues from
professional services provided on a fixed fee basis are recognized using the
percentage of completion method, measured by the percentage of labor hours
incurred to date to estimated total labor hours for each contract.

     Data Center.  We receive recurring monthly fees from financial institutions
that have chosen to use our software and outsource the processing of their
financial transactions to our data center. These fees are generally based on the
number of end users of the client institution. In addition, we receive monthly
fees for technical support. We recognize these revenues as the services are
performed.

                                       10
<PAGE>   11

                             RESULTS OF OPERATIONS

<TABLE>
<CAPTION>
                                                     FOR THE THREE MONTHS ENDED
                                       -------------------------------------------------------
                                       3/31/99    6/30/99    9/30/99    12/31/99      3/31/00
                                       -------    -------    -------    ---------    ---------
                                                           (IN THOUSANDS)
<S>                                    <C>        <C>        <C>        <C>          <C>
REVENUES:
  Software licenses..................  $ 2,308    $ 2,330    $ 2,260    $  12,152    $  10,719
  Professional services..............    7,722     10,911     14,769       23,030       34,382
  Data center........................    1,547      2,044      2,072        3,195        3,507
  Other..............................      423        390      5,698        2,039        1,761
                                       -------    -------    -------    ---------    ---------
          Total revenues.............   12,000     15,675     24,799       40,416       50,369
                                       -------    -------    -------    ---------    ---------
DIRECT COSTS:
  Software licenses..................      133         99         99          311        1,521
  Professional services..............    4,955      6,431      8,480       16,461       25,884
  Data center........................    1,687      2,029      2,163        3,129        3,352
  Other..............................      344        333      4,425        2,010        1,594
                                       -------    -------    -------    ---------    ---------
          Total direct costs.........    7,119      8,892     15,167       21,911       32,351
                                       -------    -------    -------    ---------    ---------
          Gross margin...............    4,881      6,783      9,632       18,505       18,018
                                       -------    -------    -------    ---------    ---------
OPERATING EXPENSES:
  Selling and marketing..............    1,079      1,174      1,153        8,763       11,406
  Product development................    4,321      4,439      5,221       10,055       14,992
  General and administrative.........    1,562      2,132      3,291        6,928        9,351
  Depreciation and amortization......    1,194      1,267      1,465        2,998        3,404
  Stock option compensation
     expense.........................      107        107        107          797        1,125
  Marketing cost from warrant
     issued..........................       --         --         --          715        4,600
  Merger related costs...............       --        250      1,851        6,643        6,814
  Acquired in-process research and
     development.....................       --         --         --       59,300           --
  Amortization of acquisition
     intangibles.....................      103        103         --       40,000       77,127
                                       -------    -------    -------    ---------    ---------
          Total operating expenses...    8,366      9,472     13,088      136,199      128,819
                                       -------    -------    -------    ---------    ---------
Operating loss.......................  $(3,485)   $(2,689)   $(3,456)   $(117,694)   $(110,801)
                                       =======    =======    =======    =========    =========
</TABLE>

                                       11
<PAGE>   12

                       SUMMARY INCOME STATEMENT ANALYSIS
                      (AS A PERCENTAGE OF TOTAL REVENUES)

<TABLE>
<CAPTION>
                                                            FOR THE THREE MONTHS ENDED
                                               ----------------------------------------------------
                                               3/31/99    6/30/99    9/30/99    12/31/99    3/31/00
                                               -------    -------    -------    --------    -------
<S>                                            <C>        <C>        <C>        <C>         <C>
REVENUES:
  Software licenses..........................     19%        15%         9%         30%        21%
  Professional services......................     64         70         60          57         68
  Data center................................     13         13          8           8          7
  Other......................................      4          2         23           5          4
                                                 ---        ---        ---        ----       ----
          Total revenues.....................    100        100        100         100        100
                                                 ---        ---        ---        ----       ----
DIRECT COSTS:
  Software licenses..........................      1          1         --           1          3
  Professional services......................     41         41         34          41         51
  Data center................................     14         13          9           8          7
  Other......................................      3          2         18           5          3
                                                 ---        ---        ---        ----       ----
          Total direct costs.................     59         57         61          55         64
                                                 ---        ---        ---        ----       ----
          Gross margin.......................     41         43         39          45         36
                                                 ---        ---        ---        ----       ----
OPERATING EXPENSES:
  Selling and marketing......................      9          7          5          22         22
  Product development........................     36         28         21          25         30
  General and administrative.................     13         14         13          17         19
  Depreciation and amortization..............     10          8          6           7          7
  Stock option compensation expense..........      1          1         --           2          2
  Marketing cost from warrant issued.........     --         --         --           2          9
  Merger related costs.......................     --          2          7          16         14
  Acquired in-process research and
     development.............................     --         --         --         147         --
  Amortization of acquisition intangibles....      1          1         --          99        153
                                                 ---        ---        ---        ----       ----
          Total operating expenses...........     70         61         52         337        256
                                                 ---        ---        ---        ----       ----
Operating loss...............................    (29)%      (18)%      (13)%      (292)%     (220)%
                                                 ===        ===        ===        ====       ====
</TABLE>

COMPARISON OF THE THREE MONTHS ENDED MARCH 31, 2000 AND MARCH 31, 1999

  Revenues

     S1's total revenues increased by $38.4 million to $50.4 million for the
three months ended March 31, 2000 from $12.0 million in the three months ended
March 31, 1999, an increase of 320%. The primary components of first quarter
2000 revenue were $10.7 million in software license fees, $34.4 million in
professional service fees, $3.5 million in data center fees and $1.8 million of
other revenue primarily related to third party hardware and software sales.

     Total revenue for the Internet Financial Services segment increased by
$27.6 million to $39.6 million in the first quarter 2000 compared to $12.0
million in the comparable quarter in 1999, an increase of 230%. The increase in
the first quarter of 2000 includes $12.4 million related to the companies
acquired in the fourth quarter 1999. The remaining increase of $15.2 million
relates to increases in professional services revenue, data center revenue and
other revenue discussed further below. Total revenue for the Internet
Aggregation Services segment was $314,000 in the first quarter of 2000. Total
revenue for the Call Center Technology segment for the first quarter 2000 was
$10.5 million. Both the Internet Aggregation Services and Call Center Technology
segment were established following the acquisitions of Edify and VerticalOne in
the fourth quarter 1999, therefore the entire increase relates to these
acquisitions.

                                       12
<PAGE>   13

     As a result of the size of the companies we do business with, the magnitude
of the implementations for companies of this size and our limited amount of
capacity to perform implementations, in any given period a significant portion
of our revenues may be attributed to a limited number of clients. For the three
months ended March 31, 2000, one major customer accounted for approximately 36%
of total revenues. For the three months ended March 31, 1999, three major
customers accounted for 44%, 17% and 12% of total revenue, respectively.

     Software Licenses.  Software license fees increased by $8.4 million to
$10.7 million in the three months ended March 31, 2000 from $2.3 million in the
three months ended March 31, 1999, an increase of 364%. The majority of the
increase relates to the sale of Call Center Technology products which resulted
from the acquisition of Edify which occurred in the fourth quarter of 1999.
Software license fees represented 21% of total revenues in the three months
ended March 31, 2000 compared to 19% of total revenues in the three months ended
March 31, 1999. This increase in software license fees as a percentage of
revenue is attributable to the larger concentration of license revenue in the
companies acquired during late 1999.

     Professional Services.  Professional services revenues increased by $26.7
million to $34.4 million in the three months ended March 31, 2000 from $7.7
million in the three months ended March 31, 1999, an increase of 345%. The
increase consists of $14.4 million related to the Edify and FICS acquisitions in
the fourth quarter of 1999. The approximately $12.3 million remaining increase
was driven by several large implementation projects and a large product
enhancement project. Professional services revenues represented 68% of total
revenues in the three months ended March 31, 2000 compared to 64% of total
revenues in the three months ended March 31, 1999.

     Data Center.  Data Center revenues increased by $2.0 million to $3.5
million in the three months ended March 31, 2000 from $1.5 million in the three
months ended March 31, 1999, an increase of 127%. Approximately $314,000 of the
increase is related to the acquisition of VerticalOne in the fourth quarter of
1999. The remaining $1.7 million increase is attributable to increased end-users
using S1's Internet financial applications as Data Center customers ramp up
their Internet programs. Data Center revenues represented 7% of total revenues
in the three months ended March 31, 2000 compared to 13% of total revenues in
the three months ended March 31, 1999. The decrease in Data Center revenue as a
percentage of total revenue is due to the increase in total revenue as a result
of the acquisitions during 1999. The average quarterly revenue per billable
end-user decreased to $12.43 in the first quarter 2000 from $15.99 in the first
quarter 1999. As expected, the average quarterly revenue per billable end-user
has decreased as financial services entities have increased the number of
customers using the products. Management anticipates that this trend will
continue. In addition, Data Center revenues could remain relatively flat in the
second quarter of 2000 compared to the first quarter of 2000 as a result of the
minimum fee arrangements in place with certain customers.

     Revenues associated with the data center are directly influenced by the
number of financial services entities that are using the Company's products
through the S1 Data Center and the number of end-users of these financial
services entities. During the month of March 2000, the Data Center processed in
excess of 570,000 Internet accounts, representing approximately 412,000
end-users. This represents an increase of 254% in the number of accounts
processed from approximately 161,000 and an increase of 311% in the number of
end-users from approximately 100,200 for the month of March 1999. A significant
component of the increase related to one customer who enabled approximately
100,000 of their customers on our software near the end of March. The enablement
was part of an arrangement with the customer which includes a combined fee for
professional services and Data Center services. This arrangement is part of a
strategic deployment and as a result, the fee arrangement is not based on a per
end-user per month charge. Accordingly, this arrangement did not impact revenue
in the first quarter 2000 and the effect on future quarters is currently being
evaluated.

     Other.  Other revenues, primarily related to the sale of third party
hardware and software that are used in connection with our products, increased
by $1.3 million to $1.8 million in the three months ended March 31, 2000 from
$423,000 in the three months ended March 31, 1999, an increase of 316%. Other
revenues represented 4% of total revenues in the three months ended March 31,
2000 and March 31, 1999. The dollar increase in the three months ended March 31,
2000 is due to the sale of third party hardware and

                                       13
<PAGE>   14

software primarily to two customers. We do not expect to experience this level
of other revenues in future periods.

  Direct Costs

     Direct costs increased by $25.2 million to $32.4 million in the three
months ended March 31, 2000 from $7.1 million in the three months ended March
31, 1999, an increase of 354%. Direct costs represented 64% of total revenues in
the quarter ended March 31, 2000 compared to 59% of total revenues in the
quarter ended March 31, 1999.

     Software License Costs.  Direct software license costs consist primarily of
the cost of third-party software used in the call center technology and in the
S1 Consumer Suite. Direct costs associated with software licenses increased by
$1.4 million to $1.5 million in the three months ended March 31, 2000 from
$133,000 in the three months ended March 31, 1999, an increase of 1044%. The
$1.4 million increase in direct software license costs was attributable to the
sale of the call center technology products which resulted from the acquisition
of Edify in the fourth quarter of 1999. Direct costs associated with software
licenses represented 14% of software license fees in the three months ended
March 31, 2000 compared to 6% of software license fees in the three months ended
March 31, 1999. We anticipate that direct costs associated with software
licenses as a percentage of revenue could continue to increase in future periods
as additional products are sold that include third-party software.

     Professional Services Costs.  Direct professional services costs consist
primarily of personnel and related infrastructure costs. Direct costs associated
with professional services increased by $20.9 million to $25.9 million in the
three months ended March 31, 2000 from $5.0 million in the three months ended
March 31, 1999, an increase of 422%. Approximately $13.0 million of the increase
in professional services costs is attributable to the 1999 acquisitions. The
remaining increase of $7.9 million relates to additional resources needed to
perform work on the large implementations and product enhancement projects.
Direct costs associated with professional services represented 75% of
professional services revenues in the three months ended March 31, 2000,
compared to 64% of professional services revenues in the three months ended
March 31, 1999. This decrease is primarily due to lower margin contracts
inherited as part of the acquisitions made in 1999. We anticipate that
professional services margins could decrease for the next several quarters as
the acquired companies historically experienced lower professional services
margins than we experienced on a stand-alone basis.

     Data Center Costs.  Direct data center costs consist of personnel and
computer equipment costs. Direct costs associated with data center services
increased by $1.7 to $3.4 million in the three months ended March 31, 2000 from
$1.7 million in the three months ended March 31, 1999, an increase of 99%.
Approximately $200,000 of the increase relates to the operation of the Vertical
One data center. The increase in direct costs relates to the opening of the new
data center in the third quarter of 1999. The remaining increase is also
attributed to equipment costs to support new financial institutions and upgraded
and new applications. Direct data center costs represented 96% of data center
revenues in the three months ended March 31, 2000, compared to 109% of data
center revenues in the three months ended March 31, 1999. We expect the direct
data center costs to increase in future quarters as our data centers in Asia and
Europe are brought into production.

     Other Direct Costs.  Other direct costs consist primarily of the cost of
third party software and hardware sold for use with our products. Other direct
costs increased by $1.3 million to $1.6 million in the three months ended March
31, 2000 from $344,000 in the three months ended March 31, 1999, an increase of
363%. Other direct costs represented 91% of other revenues in the three months
ended March 31, 2000, compared to 81% of other revenue in the three months ended
March 31, 1999.

  Operating Expenses

     Operating expenses increased by $120.5 million to $128.8 million in the
three months ended March 31, 2000 from $8.4 million in the three months ended
March 31, 1999.

                                       14
<PAGE>   15

     Selling and Marketing.  Selling and marketing expenses increased by $10.3
million to $11.4 million in three months ended March 31, 2000 from $1.1 million
in the three months ended March 31, 1999, an increase of 957%. The acquisitions
in late 1999 accounted for approximately $9.1 million of the increase in selling
and marketing expense. The remaining increase in selling and marketing expense
for the three months ended March 31, 2000 is primarily due to an increase in
personnel to manage the larger customer relationships. Selling and marketing
expenses were 23% of total revenues in the three months ended March 31, 2000,
compared to 9% of total revenues in the three months ended March 31, 1999. The
percentage increase was due primarily to the acquisition of Edify during the
fourth quarter of 1999. Edify's sales model was targeted at a larger number of
smaller financial institutions, resulting in higher sales expense as a result of
having a larger sales force.

     Product Development.  Product development expenses increased by $10.7
million to $15.0 million in the three months ended March 31, 2000 from $4.3
million in the three months ended March 31, 1999, an increase of 247%. Product
development expenses were 30% of total revenues in the three months ended March
31, 2000, compared to 36% of total revenues in the three months ended March 31,
1999. Approximately $9.1 million of the dollar increase relates to the
acquisitions in the fourth quarter of 1999. The remainder of the dollar increase
in product development costs represents our efforts to enhance the current
products by migrating the existing products to a more efficient software
architecture and to developing new applications. The decrease as a percentage of
our total revenues resulted from our ability to leverage product development
expenses over a larger revenue base.

     General and Administrative.  General and administrative expenses increased
by $7.8 million to $9.4 million in the three months ended March 31, 2000 from
$1.6 million in the three months ended March 31, 1999, an increase of 499%.
General and administrative expenses were 19% of total revenues in the three
months ended March 31, 2000 compared to 13% of total revenues in the three
months ended March 31, 1999. Approximately $4.8 million of the increase relates
to the acquired companies. The remaining increase relates to the expanded
personnel infrastructure to manage our growth and global operations.

     Depreciation and Amortization.  Depreciation and amortization expenses
increased by $2.2 million to $3.4 million in the three months ended March 31,
2000 from $1.2 million in the three months ended March 31, 1999, an increase of
185%. Depreciation and amortization expenses were 7% of total revenues for the
three months ended March 31, 2000, compared to 10% of total revenues in the
three months ended March 31, 1999. Approximately $1.7 million of the increase
related to the 1999 acquisitions. The remainder of the increase is due to the
completion of the Data Center in third quarter 1999.

     Stock Option Compensation Expense.  Stock option compensation expense
increased by $1.0 million to $1.1 million for the three months ended March 31,
2000 compared to $107,000 for the three months ended March 31, 1999, an increase
of 935%. The increase is the result of the acquisition of VerticalOne in the
fourth quarter of 1999. Stock option compensation expense represents the
difference between the exercise price of stock options granted and the fair
value of our common stock at the time of grant. This amount is being amortized
over the vesting periods of the applicable options.

     Marketing Cost for Warrant Issued.  The Company recorded $4.6 million of
marketing costs in the three months ended March 31, 2000 related to the issuance
to a third-party of a stock warrant to purchase 84,994 shares of our common
stock at an exercise price of $80.00 per share. The warrant was issued in
connection with a pilot project and distribution agreement between the
third-party and one of our subsidiaries. The fair value of the warrant was
determined based on the Black-Scholes option-pricing model. This amount was
amortized over the period of the pilot project and distribution agreement, which
was approximately 60 days.

     Merger Related Costs.  Merger related costs for the three months ended
March 31, 2000 were comprised of $6.8 million related to the acquisitions of
Edify, FICS, and VerticalOne. Approximately $1.8 million of merger related costs
incurred in the first quarter of 2000 consist of the cost of product development
personnel working on the integration of all the products and platforms of the
acquired companies. Also included in merger related costs incurred in the three
months ended March 31, 2000 are approximately $1.4 million of personnel costs
related to professional service employees who are being cross trained on the
products and services of the acquired companies. After the integration efforts
are complete, these costs will be captured in
                                       15
<PAGE>   16

the product development and professional services expense categories in the
income statement. The remaining merger related costs incurred in the first
quarter of 2000 consist primarily of expenditures related to building and
consolidating infrastructure and other operations of the companies we acquired
in the fourth quarter of 1999.

     Amortization of Goodwill and Acquisition Charges.  Amortization of goodwill
and acquisition charges increased $77.0 million to $77.1 million in the three
months ended March 31, 2000 from $103,000 in the three months ended March 31,
1999, an increase of 74781%. Amortization of goodwill and acquisition charges
were 153% of total revenues in the three months ended March 31, 2000 compared to
1% of total revenues in the three months ended March 31, 1999. The increase
related to the amortization of goodwill and other identifiable intangible assets
resulting from the acquisitions of Edify, FICS, and VerticalOne in the fourth
quarter of 1999.

LIQUIDITY AND CAPITAL RESOURCES

     Total stockholders' equity decreased to $907.2 million as of March 31, 2000
from $990.8 million at December 31, 1999. The decrease in stockholders' equity
is attributable to the net loss of $75.2 million for the three months ended
March 31, 2000 and the realized gain on the sale of investment securities
available for sale, offset by the proceeds from the sale of common stock under
the Employee Stock Purchase and Option Plans, stock compensation expense, and an
increase in additional paid-in capital resulting for the warrant issued for
marketing.

     As of March 31, 2000, we had cash and cash equivalents of $75.1 million
compared to $67.9 million at December 31, 1999 and investment securities
available for sale, which consisted entirely of equity securities, of $38.5
million at March 31, 2000 compared to $62.8 million at December 31, 1999.

     In the first quarter of 2000, cash used in operations was $21.4 million
compared to cash provided by operations of $7.6 million in the comparable
quarter of 1999. The increase in cash used in operations between 2000 and 1999
is primarily the result of an increase in the net loss.

     Cash provided by investing activities was $22.1 million in the first
quarter of 2000 compared to net cash used in investing activities of $0.7
million in the comparable quarter of 1999. The increase in cash provided by
investing activities in the first quarter of 2000 was the result of the proceeds
from the sale of investment securities available for sale of $36.5 million,
which was offset by purchases of property and equipment of $8.9 million. Capital
expenditures in the first quarter of 2000 relate to the expansion of our
facilities infrastructure to accommodate our growth and the establishment of our
new Data Center facilities in Asia and Europe. During the quarter, investments
and advances of approximately $5.5 million were made in companies that are
considered to have technology or products that compliment our product and
services offerings. We expect to continue to make strategic investments in
companies that have applications or platforms that are synergistic to our
product offerings.

     Cash provided by financing activities was $6.7 million in the first quarter
of 2000 compared to $1.5 million in comparable quarter of 1999. The increase
relates to proceeds of $9.2 million from the sale of common stock under the
Employee Stock Purchase and Option Plans in the first quarter of 2000 compared
to $1.0 million in the comparable quarter of 1999. These increases were offset
by payments made on notes payable of $2.4 million in the first quarter of 2000.

     Basic loss per share is calculated as income available to common
stockholders divided by the weighted average number of common shares outstanding
during the period. Diluted earnings per share is calculated to reflect the
potential dilution that would occur if stock options or other contracts to issue
common stock were exercised and resulted in additional common stock that would
share in the earnings of the Company. Because of the Company's net losses, the
issuance of additional shares of common stock under stock options and warrants
or upon the conversion of preferred stock would be antidilutive. The total
number of common shares that would have been used in the Company's computation
of diluted earnings per share for the three month period ended March 31, 2000
and 1999 was 66,757,024 and 36,407,070 respectively. As discussed in Note 2,

                                       16
<PAGE>   17

S1 closed the transactions with Q Up and Davidge on April 7 and 13, 2000,
respectively. These acquisitions could have a dilutive effect on earnings per
share in future periods.

RECENT ACCOUNTING PRONOUNCEMENTS

     In December 1999, the SEC issued Staff Accounting Bulletin No. 101,
"Revenue Recognition in Financial Statements" ("SAB101"). In March 2000, the SEC
released SAB101A which defers reporting on the effects of the adoption of SAB
101 until the second fiscal quarter of 2000. The Company is evaluating the
implications of SAB 101 and to date nothing has been identified that would
significantly change the Company's historical practices with respect to the
events that trigger revenue recognition and the measurement of revenue.

ITEM 3.  QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

     Quantitative and qualitative disclosures about market risk were included in
Item 7A of the Company's 1999 Annual Report on Form 10-K. There have been no
significant changes in the Company's market risk from December 31, 1999.

                          PART II -- OTHER INFORMATION

ITEM 1.  LEGAL PROCEEDINGS.

     On May 8, 2000, three identical complaints were filed in the United States
District Court in Atlanta, Georgia by Amy Zimmerman, Shoshana Hirsch and Bruce
McCall, respectively, against S1, Michel Akkermans, James Mahan and Robert
Stockwell alleging violations of Section 10(b) of the Securities Exchange Act of
1934 and Rule 10b-5 promulgated thereunder. Each of the plaintiffs claims to
have purchased 100 shares of S1 stock between February and April 2000, and each
seeks to represent a class of all persons who purchased S1 shares between
November 2, 1999 and May 2, 2000. Their complaints assert generally that S1 and
the individual defendants made or authorized the issuance of false and
misleading statements concerning the anticipated performance of S1, which they
claim led to artificial inflation of the price at which S1's shares were traded.
S1 believes that the allegations in these complaints are wholly devoid of merit,
and it and the individual defendants intend to defend themselves vigorously
against these claims.

ITEM 2.  CHANGES IN SECURITIES AND USE OF PROCEEDS.

     (a) Not applicable.

     (b) Not applicable.

     (c) On March 28, 2000, S1 issued to The Credo Group 1,903 shares of S1's
         common stock pursuant to a letter agreement dated August 29, 1999
         between VerticalOne Corporation and The Credo Group as partial payment
         for certain services provided to VerticalOne by The Credo Group. S1
         also issued 97 shares of S1 common stock in regard to this issuance
         that will be held in escrow until November 2000 in connection with
         November 1999 acquisition of VerticalOne Corporation by S1. The offer
         and sale of the stock satisfied the requirements of Section 4(2) of the
         Securities Act of 1933, as amended (transactions by an issuer not
         involving any public offering).

     (d) Not applicable.

ITEM 3.  DEFAULTS UPON SENIOR SECURITIES.

     Not applicable.

ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS.

     Not applicable.

                                       17
<PAGE>   18

ITEM 5.  OTHER INFORMATION.

     None.

ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K.

     (a) Exhibits.

<TABLE>
<CAPTION>
EXHIBIT
  NO.                             DESCRIPTION
-------                           -----------
<C>       <S>
  4       Specimen certificate for S1 Corporation common stock.
 10.1     Agreement and Plan of Merger, dated as of March 6, 2000, by
          and among S1, Austin Acquisition Corporation, Q-Up Systems,
          Inc., and for the limited purposes set forth therein certain
          of the Q-Up shareholders who are signatories thereto.
 10.2     Amendment No. 1 to Agreement and Plan of Merger, dated as of
          March 15, 2000, by and among S1, Austin Acquisition
          Corporation, Q-Up Systems, Inc., and the shareholders of
          Q-Up who are signatories thereto.
 10.3     Amendment to Security First Technologies Corporation Amended
          and Restated 1995 Stock Option Plan.
 27       Financial Data Schedule.
</TABLE>

     (b) Reports on Form 8-K.

     S1 filed the following Current Report on Form 8-K with the Securities and
Exchange Commission (the "SEC") during the quarter ended March 31, 2000:

          Current Report on Form 8-K filed with the SEC on March 8, 2000 (date
     of report March 6, 2000) (announcing the proposed acquisition of Q Up
     Systems, Inc. by S1).

                                       18
<PAGE>   19

                                   SIGNATURES

     Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized, as of May 15, 2000.

                                          S1 CORPORATION

                                          By: /s/ ROBERT F. STOCKWELL
                                            ------------------------------------
                                            Robert F. Stockwell
                                            Chief Financial Officer
                                            (Principal Financial Officer and
                                            Principal Accounting Officer)

                                       19
<PAGE>   20

                                 EXHIBIT INDEX

<TABLE>
<CAPTION>
EXHIBIT
  NO.                            DESCRIPTION
-------                          -----------
<C>      <S>
  4      Specimen certificate for S1 Corporation common stock.
 10.1    Agreement and Plan of Merger, dated as of March 6, 2000, by
         and among S1, Austin Acquisition Corporation, Q-Up Systems,
         Inc., and for the limited purposes set forth therein certain
         of the Q-Up shareholders who are signatories thereto.
 10.2    Amendment No. 1 to Agreement and Plan of Merger, dated as of
         March 15, 2000, by and among S1, Austin Acquisition
         Corporation, Q-Up Systems, Inc., and the shareholders of
         Q-Up who are signatories thereto.
 10.3    Amendment to Security First Technologies Corporation Amended
         and Restated 1995 Stock Option Plan.
 27      Financial Data Schedule.
</TABLE>