<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                   FORM 10-K/A
                                 AMENDMENT NO. 1

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

For the Fiscal Year Ended                                Commission file number
     December 31, 2004                                           0-16759

                           FIRST FINANCIAL CORPORATION
             (Exact name of registrant as specified in its charter)

              INDIANA                                    35-1546989
      (State of Incorporation)              (I.R.S. Employer Identification No.)

      One First Financial Plaza                            47807
          Terre Haute, IN
(Address of principal executive offices)                 (Zip Code)

                  Registrant's telephone number: (812) 238-6000

        Securities registered pursuant to Section 12(b) of the Act: None

           Securities registered pursuant to Section 12(g) of the Act:

        TITLE OF EACH CLASS          NAME OF EACH EXCHANGE ON WHICH REGISTERED
    --------------------------       -----------------------------------------
    Common Stock, no par value                       Nasdaq

     Indicated 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, and (2) has been subject to such filing
requirements for the past 90 days. Yes [X] No [ ]

     Indicate by check mark if disclosure of delinquent filers pursuant to item
405 of regulation S-K is not contained herein, and will not be contained, to the
Registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of the Form 10-K or any amendment to the
form 10-K. [X]

     Indicate by check mark whether the registrant is an accelerated filer (as
defined in Rule 12b-2 of the Act) Yes [X] No [ ]

     As of June 30, 2004 the aggregate market value of the voting stock held by
nonaffiliated of the registrant based on the average bid and ask prices of such
stock was $393,720,021. (For purposes of this calculation, the Corporation
excluded the stock owned by certain beneficial owners and management and the
Corporation's ESOP.)

     Shares of Common Stock outstanding as of March 9, 2005--13,505,938 shares.

                       DOCUMENTS INCORPORATED BY REFERENCE

     Portions of the 2004 Annual Report to Shareholders are incorporated by
reference into Parts I and II. Portions of the Definitive Proxy Statement for
the First Financial Corporation Annual Meeting of Shareholders held on April 20,
2005 are incorporated by reference into Part III.

<PAGE>

                                EXPLANATORY NOTE

     In reliance upon the Order of the Securities and Exchange Commission issued
under Section 36 of the Securities Exchange Act of 1934 (Release No. 50754,
November 30, 2004), First Financial Corporation (the "Corporation") omitted from
Item 9A of its Annual Report on Form 10-K for the fiscal year ended December 31,
2004 originally filed on March 16, 2005 (the "Original Report") both
Management's annual report on internal control over financial reporting, as
required by Item 308(a) of Regulation S-K, as well as the related Attestation
report of the registered public accounting firm, as required by Item 308(b) of
Regulation S-K.

     This Amendment No. 1 to Annual Report on Form 10-K (the "Amendment") is
specifically intended to make only the following changes to the Corporation's
10-K for the year ended December 31, 2004: (1) to modify Part II Item 9A to
include the information required by Items 308(a) and 308(b) of Regulation S-K;
(2) to update the requisite Section 302 and Section 906 certifications and the
signature page to reflect the filing of this Amendment; and (3) to modify the
list of exhibits to reflect the updated certifications and to incorporate by
reference any other exhibits filed with the Original Filing. All other
information presented in the Original Filing is unaffected by this Amendment,
and has thus not been reiterated herein. All information in this Amendment is as
of the date of the Original Filing and does not reflect any subsequent
information or events occurring after the date of the Original Filing.

ITEM 9A. CONTROLS AND PROCEDURES (AMENDED).

EVALUATION OF DISCLOSURE CONTROLS AND PROCEDURES

As of the end of the period covered by this report, we carried out an evaluation
(the "Evaluation"), under the supervision and with the participation of our
President and Chief Executive Officer ("CEO"), who serves as our principal
executive officer, and Chief Financial Officer ("CFO"), who serves as our
principal financial officer, of the effectiveness of our disclosure controls and
procedures ("Disclosure Controls"). Based on the Evaluation, our CEO and CFO
concluded that our Disclosure Controls are effective in alerting them in a
timely way to material information required to be included in our periodic SEC
reports.

MANAGEMENT'S REPORT ON INTERNAL CONTROL OVER FINANCIAL REPORTING

The management of First Financial Corporation (the "Corporation") has prepared
and is responsible for the preparation and accuracy of the consolidated
financial statements and related financial information included in the Annual
Report. The consolidated financial statements have been prepared in accordance
with accounting principles generally accepted in the United States of America
and include amounts based on judgments and estimates by management.

The management of the Corporation is responsible for establishing and
maintaining adequate internal control over financial reporting as defined in
Rules 13a-15(f) and 15d-15(f) under the Securities Exchange Act of 1934. The
Corporation's internal control over financial reporting is designed to provide
reasonable assurance regarding the reliability of financial reporting and the
preparation of financial statements for external purposes in accordance with
generally accepted accounting principles. The Corporation's internal control
over financial reporting includes those policies and procedures that: (i)
pertain to the maintenance of records that, in reasonable detail, accurately and
fairly reflect the transactions and dispositions of the assets of the
Corporation; (ii) provide reasonable assurance that transactions are recorded as
necessary to permit preparation of financial statements in accordance with
generally accepted accounting principles, and that receipts and expenditures of
the Corporation are being made only in accordance with authorizations of
management and directors of the Corporation; and (iii) provide reasonable
assurance regarding prevention or

                                                                               2

<PAGE>

timely detection of unauthorized acquisition, use or disposition of the
Corporation's assets that could have a material effect on the financial
statements.

The system of internal control over financial reporting as it relates to the
consolidated financial statements is evaluated for effectiveness by management
and tested for reliability through a program of internal audits. Because of its
inherent limitations, internal control over financial reporting may not prevent
or detect misstatements. Also, projections of any evaluation of effectiveness to
future periods are subject to the risk that controls may become inadequate
because of changes in conditions, or that the degree of compliance with the
policies or procedures may deteriorate.

Management assessed the Corporation's system of internal control over financial
reporting as of December 31, 2004, in relation to criteria for effective
internal control over financial reporting as described in "Internal Control -
Integrated Framework," issued by the Committee of Sponsoring Organizations of
the Treadway Commission. Based on this assessment, management concluded that, as
of December 31, 2004, its system of internal control over financial reporting is
effective and meets the criteria of the "Internal Control - Integrated
Framework".

Crowe Chizek and Company LLC, independent registered public accounting firm, has
issued an attestation report dated April 15, 2005 on management's assessment of
the Corporation's internal control over financial reporting.

CHANGES IN INTERNAL CONTROLS

There was no change in the Corporation's internal control over financial
reporting that occurred during the Corporation's fourth fiscal quarter of 2004
that has materially affected, or is reasonably likely to materially affect, the
Corporation's internal control over financial reporting.

REPORT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM ON INTERNAL CONTROL OVER
FINANCIAL REPORTING

Shareholders and Board of Directors
First Financial Corporation
Terre Haute, Indiana

We have audited management's assessment, included in the accompanying
Management's Report on Internal Control Over Financial Reporting, that First
Financial Corporation maintained effective internal control over financial
reporting as of December 31, 2004, based on criteria established in Internal
Control--Integrated Framework issued by the Committee of Sponsoring
Organizations of the Treadway Commission (COSO). First Financial Corporation's
management is responsible for maintaining effective internal control over
financial reporting and for its assessment of the effectiveness of internal
control over financial reporting. Our responsibility is to express an opinion on
management's assessment and an opinion on the effectiveness of the company's
internal control over financial reporting based on our audit.

We conducted our audit in accordance with the standards of the Public Company
Accounting Oversight Board (United States). Those standards require that we plan
and perform the audit to obtain reasonable assurance about whether effective
internal control over financial reporting was maintained in all material
respects. Our audit included obtaining an understanding of internal control over
financial reporting, evaluating management's assessment, testing and evaluating
the design and operating effectiveness of internal control, and performing such
other procedures as we considered necessary in the circumstances. We

                                                                               3

<PAGE>

believe that our audit provides a reasonable basis for our opinion.

A company's internal control over financial reporting is a process designed to
provide reasonable assurance regarding the reliability of financial reporting
and the preparation of financial statements for external purposes in accordance
with generally accepted accounting principles. A company's internal control over
financial reporting includes those policies and procedures that (1) pertain to
the maintenance of records that, in reasonable detail, accurately and fairly
reflect the transactions and dispositions of the assets of the company; (2)
provide reasonable assurance that transactions are recorded as necessary to
permit preparation of financial statements in accordance with generally accepted
accounting principles, and that receipts and expenditures of the company are
being made only in accordance with authorizations of management and directors of
the company; and (3) provide reasonable assurance regarding prevention or timely
detection of unauthorized acquisition, use, or disposition of the company's
assets that could have a material effect on the financial statements.

Because of its inherent limitations, internal control over financial reporting
may not prevent or detect misstatements. Also, projections of any evaluation of
effectiveness to future periods are subject to the risk that controls may become
inadequate because of changes in conditions, or that the degree of compliance
with the policies or procedures may deteriorate.

In our opinion, management's assessment that First Financial Corporation
maintained effective internal control over financial reporting as of December
31, 2004, is fairly stated, in all material respects, based on criteria
established in Internal Control--Integrated Framework issued by the Committee of
Sponsoring Organizations of the Treadway Commission (COSO). Also in our opinion,
First Financial Corporation maintained, in all material respects, effective
internal control over financial reporting as of December 31, 2004, based
criteria established in Internal Control--Integrated Framework issued by the
Committee of Sponsoring Organizations of the Treadway Commission (COSO).

We have also audited, in accordance with the standards of the Public Company
Accounting Oversight Board (United States), the consolidated balance sheets of
First Financial Corporation as of December 31, 2004 and 2003, and the related
consolidated statements of income, changes in shareholders' equity, and cash
flows for each of the three years in the period ended December 31, 2004 and our
report dated March 8, 2005 expressed an unqualified opinion on those
consolidated financial statements.

                                             Crowe Chizek and Company LLC

Indianapolis, Indiana
April 15, 2005

                                                                               4

<PAGE>

                                     PART IV

ITEM 15. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

 (a)(1) The following consolidated financial statements of the Registrant and
             its subsidiaries are included in the 2004 Annual Report to
             Shareholders of First Financial Corporation attached:

             Consolidated Balance Sheets--December 31, 2004 and 2003

             Consolidated Statements of Income--Years ended December 31, 2004,
             2003, and 2002

             Consolidated Statements of Changes in Shareholders' Equity--Years
             ended December 31, 2004, 2003, and 2002

             Consolidated Statements of Cash Flows--Years ended December 31,
             2004, 2003, and 2002

             Notes to Consolidated Financial Statements

    (2)      Schedules to the Consolidated Financial Statements required by
             Article 9 of Regulation S-X are not required, inapplicable, or
             the required information has been disclosed elsewhere.

    (3)      Listing of Exhibits:

<TABLE>
<CAPTION>
Exhibit Number                      Description
<S>               <C>
    3.1           Amended and Restated Articles of Incorporation of First
                  Financial Corporation, incorporated by reference to
                  Exhibit 3(i) of the Corporation's Form 10-Q filed for the
                  quarter ended September 30, 2002

    3.2           Code of By-Laws of First Financial Corporation, incorporated
                  by reference to Exhibit 3(U) of the Corporation's Form 10-Q
                  filed for the quarter ended September 30, 2002

    10.1          Employment Agreement for Norman L. Lowery, dated January 1,
                  2004, incorporated by reference to Exhibit 10.2 to the
                  Corporation's Form 10-Q filed for the quarter ended March
                  31, 2004

    10.2          2001 Long-Term Incentive Plan of First Financial Corporation,
                  incorporated by reference to Exhibit 10.3 of the Corporation's
                  Form 10--Q filed for the quarter ended September 30, 2002

    10.3          2005 Schedule of Director Compensation

    10.4          2005 Schedule of Named Executive Officer Compensation

    13            Annual Report

    21            Subsidiaries

    31.1          Certification pursuant to Rule 13a-14(a) for Annual Report of
                  Form 10-K by Principal Executive Officer

    31.2          Certification pursuant to Rule 13a-14(a) for Annual Report of
                  Form 10-K by Principal Financial Officer

    32.1          Certification pursuant to 18 U.S.C. Section 1350 of Principal
                  Executive Officer

    32.2          Certification pursuant to 18 U.S.C. Section 1350 of Principal
                  Financial Officer
</TABLE>

(b)  Exhibits-Exhibits to (a) (3) listed above are attached to this report.

(c)  Financial Statements Schedules-No schedules are required to be submitted.
     See response to ITEM 15(a) (2).

                                                                               5

<PAGE>

                                   SIGNATURES

     Pursuant to the requirements of Section 13 or 15(d) 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.

                                  First Financial Corporation

Dated:  April 26, 2005            Michael A. Carty, signed
                                  ---------------------------------------------
                                  Michael A. Carty, Secretary, Treasurer & CFO
                                  (Principal Financial Officer and Principal
                                  Accounting Officer)

Pursuant to the requirements of the Securities Exchange Act of 1934, this report
has been signed below by the following persons on behalf of the registrant and
in the capacities and on the dates indicated.

<TABLE>
<CAPTION>
          NAME                                                 DATE
<S>                                                       <C>
Donald E. Smith, signed                                   April 26, 2005
-------------------------------------------------
Donald E. Smith, President and Director

Michael A. Carty, signed                                  April 26, 2005
-------------------------------------------------
Michael A. Carty, Secretary, Treasurer & CFO
(Principal Financial Officer and Principal
 Accounting Officer)

W. Curtis Brighton, signed                                April 26, 2005
-------------------------------------------------
W. Curtis Brighton, Director

B. Guille Cox, Jr., signed                                April 26, 2005
-------------------------------------------------
B. Guille Cox, Jr., Director

Thomas T. Dinkel, signed                                  April 26, 2005
-------------------------------------------------
Thomas T. Dinkel, Director

Anton H. George, signed                                   April 26, 2005
-------------------------------------------------
Anton H. George, Director

Gregory L. Gibson, signed                                 April 26, 2005
-------------------------------------------------
Gregory L. Gibson, Director

Norman L. Lowery, signed                                  April 26, 2005
-------------------------------------------------
Norman L. Lowery, Vice Chairman, CEO & Director
(Principal Executive Officer)

William A. Niemeyer, signed                               April 26, 2005
-------------------------------------------------
William A. Niemeyer, Director

Patrick O'Leary, signed                                   April 26, 2005
-------------------------------------------------
Patrick O'Leary, Director

                                                          April 26, 2005
Chapman J. Root II, Director

Virginia L. Smith, signed                                 April 26, 2005
-------------------------------------------------
Virginia L. Smith, Director
</TABLE>

                                                                               6