<PAGE>   1
                       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       June 30, 1998
                                    ---------------------------


[   ] 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     0-15902
                        --------------------------------------------------------

                               ESSEF Corporation
--------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)

            Ohio                                   34-0777631
--------------------------------------------------------------------------------
(State or other jurisdiction of                 (I.R.S. Employer
 incorporation or organization)                Identification No.)

    220 Park Drive, Chardon, Ohio                        44024       
--------------------------------------------------------------------------------
(Address of principal executive offices)               (Zip Code)

Registrant's telephone number, including area code  (440) 286-2200
                                                   -----------------------------

                                      None
--------------------------------------------------------------------------------
Former name, former address and former fiscal year, 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, as
amended, 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       N/A
                                -----      -----      -----

Indicate the number of shares outstanding of each of the issuer's classes of
common shares, as of the latest practicable date.


            Class                     Outstanding at August 11,1998
---------------------------           -----------------------------
Common Shares, no par value                11,766,341 Shares


                                  Page 1 of 13

<PAGE>   2


                                ESSEF CORPORATION
                                    FORM 10-Q

                         FOR QUARTER ENDED JUNE 30, 1998

                                      INDEX

<TABLE>
<CAPTION>
                                                                      Sequential
                                                                       Page No.
                                                                       --------

<S>                                                                       <C>
Part I - Financial Information
     Item 1.  Financial Statements
          Condensed Consolidated Balance Sheets -
                June 30, 1998 and September 30, 1997 ................      3
          Condensed Consolidated Statements of Income -
                Three Months and Nine Months Ended June 30, 1998
                    and 1997 ........................................      4
          Condensed Consolidated Statements of Cash Flows -
                    Nine Months Ended June 30, 1998 and 1997 ........      5
          Notes to Condensed Consolidated Financial
                    Statements ......................................      6-8

     Item 2.  Management's Discussion and Analysis of
              Financial Condition and Results of Operations..........    9-11

Part II - Other Information

     Item 1.  Legal Proceedings .....................................     12

     Item 2.  Changes in Securities .................................     12

     Item 4.  Submission of Matters to a Vote of Security
                  Holders ...........................................     12

     Item 6.  Exhibits and Reports on Form 8-K ......................     12
</TABLE>






                                  Page 2 of 13



<PAGE>   3



PART I.  FINANCIAL INFORMATION

ITEM 1.  FINANCIAL STATEMENTS

                       ESSEF CORPORATION AND SUBSIDIARIES
                      CONDENSED CONSOLIDATED BALANCE SHEETS
                             (Dollars in thousands)



                                                       June 30,    September 30,
                                                        1998          1997
                                                     -----------   -------------
ASSETS                                               (unaudited)    (audited)
------

Current Assets
      Cash and cash equivalents ...............     $     477      $   1,668
      Accounts receivable, net ................        64,684         39,512
      Inventories, net ........................        47,334         34,597
      Prepayments and other ...................         2,473          2,173
                                                    ---------      ---------
           Total current assets ...............       114,968         77,950

Property, plant and equipment, net ............        69,190         63,820
Goodwill, net .................................        66,545         60,349
Deferred income taxes .........................         5,576          5,706
Other .........................................        11,185          9,058
                                                    ---------      ---------
                                                    $ 267,464      $ 216,883
                                                    =========      =========


LIABILITIES AND SHAREHOLDERS' EQUITY
------------------------------------

Current Liabilities
        Short-term borrowings .................     $   5,641      $   5,320
        Current maturities of long-term debt ..         1,038            549
        Accounts payable ......................        29,906         20,028
        Accrued expenses ......................        29,869         29,237
        Accrued income taxes ..................         9,782          8,668
                                                    ---------      ---------
           Total current liabilities ..........        76,236         63,802

Long-term debt ................................       108,822         81,658
Other long-term Liabilities ...................         5,109          5,977

Shareholders' Equity
        Preferred shares without par value,
           authorized 1,000,000 shares,
           none issued ........................         -----          -----
        Common shares without par value,
           authorized 40,000,000 shares, issued
           12,264,286 and 12,149,394 shares,
           respectively .......................        32,911         32,234
           503,927 Treasury shares at cost ....        (7,962)        (7,962)
        Retained earnings .....................        52,356         41,099
        Foreign currency translation adjustment            (8)            75
                                                    ---------      ---------
                Total shareholders' equity ....        76,424         65,446
                                                    ---------      ---------
                                                    $ 267,464      $ 216,883
                                                    =========      =========



See notes to condensed consolidated financial statements.


                                  Page 3 of 13
<PAGE>   4



                       ESSEF CORPORATION AND SUBSIDIARIES
                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                      (In thousands, except per share data)
                                   (Unaudited)



                                  Three Months Ended      Nine Months Ended
                                       June 30,                June 30,
                                   1998        1997        1998        1997
                                   ----        ----        ----        ----

Net sales....................   $137,625    $102,652    $308,999    $198,460

Cost of sales................     96,143      72,411     224,251     142,937
                                --------    --------    --------    --------

  Gross profit...............     41,482      30,241      84,748      55,523

Operating expenses...........     25,087      18,746      61,378      37,345
                                --------    --------    --------    --------

  Income from operations.....     16,395      11,495      23,370      18,178

Interest and other expense...      2,287       1,643       6,052       2,908
                                --------    --------    --------    --------

  Income before income taxes.     14,108       9,852      17,318      15,270

Provision for income taxes...      4,938       3,448       6,061       5,344
                                --------    --------    --------    --------

  Net income.................   $  9,170    $  6,404    $ 11,257    $  9,926
                                ========    ========    ========    ========


Earnings per share:

  Basic (a)                         $.78        $.55        $.96        $.85
                                 =======     =======     =======     =======

  Diluted (a)                       $.68        $.49        $.84        $.76
                                 =======     =======     =======     =======



Average shares outstanding:

  Basic                           11,743      11,628      11,702      11,625
                                 =======     =======     =======     =======

  Diluted                         13,582      13,189      13,461      13,137
                                 =======     =======     =======     =======




a)       Basic and diluted earnings per share have been reported in accordance
         with SFAS Number 128, "Earnings Per Share", which was adopted by the
         Company effective October 1, 1997. Accordingly, all prior periods have
         been restated.


See notes to condensed consolidated financial statements.



                                  Page 4 of 13

<PAGE>   5



                       ESSEF CORPORATION AND SUBSIDIARIES
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                 (In thousands)
                                   (Unaudited)
<TABLE>
<CAPTION>
                                                            Nine Months Ended
                                                                  June 30,
                                                           1998              1997
                                                           ----              ----
<S>                                                        <C>           <C>
Cash Flows from Operating Activities
      Net income .....................................     $ 11,257      $  9,926
      Adjustments to reconcile net income to net
      cash provided by/(used in) operating activities
                Depreciation and amortization ........        8,903         4,730
                Other ................................         (537)          191
      Changes in operating assets and liabilities
           Accounts receivable .......................      (22,462)       (5,042)
           Inventories ...............................       (9,031)       (3,891)
           Prepayments and other assets ..............         (297)         (693)
           Accounts payable ..........................        7,079         2,613
           Accrued expenses ..........................          126        (1,475)
           Accrued and deferred income taxes .........        1,250         3,440
                                                           --------      --------
                Net cash provided by/(used in)
                 operating activities ................       (3,712)        9,799
                                                           --------      --------

Cash Flows from Investing Activities
      Additions to property, plant and
           equipment .................................      (12,806)      (10,073)
      Business acquisitions ..........................      (10,102)      (69,830)
      Other, net .....................................       (2,758)        4,453
                                                           --------      --------
                Net cash used in investing activities.      (25,666)      (75,450)
                                                           --------      --------

Cash Flows from Financing Activities
      Proceeds from long term debt ...................       27,653        70,924
      Increase(decrease) in short-term borrowings ....          321        (1,047)
      Proceeds from exercise of stock options ........          213            87
                                                           --------      --------
           Net cash provided by financing activities .       28,187        69,964
                                                           --------      --------

Net increase (decrease) in cash and
           cash equivalents ..........................       (1,191)        4,313

Cash and Cash Equivalents
  Beginning of period ................................        1,668         2,620
                                                           --------      --------
  End of period ......................................     $    477      $  6,933
                                                           ========      ========


Supplemental Cash Flow Information
           Interest paid .............................     $  5,724      $  2,708
                                                           ========      ========
           Income taxes paid .........................     $  4,607      $  1,705
                                                           ========      ========
</TABLE>





See notes to condensed consolidated financial statements.



                                  Page 5 of 13

<PAGE>   6

                       ESSEF CORPORATION AND SUBSIDIARIES
              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS


(1)        The accompanying unaudited condensed consolidated financial
           statements contain all adjustments (consisting of only normal and
           recurring adjustments) which, in the opinion of management, are
           necessary to present fairly the consolidated financial position of
           Essef Corporation and subsidiaries (the "Company") as of June 30,
           1998, and the results of operations for the three-month and
           nine-month periods ended June 30, 1998 and 1997, and cash flows for
           the nine-month periods ended June 30, 1998 and 1997.

           These condensed consolidated financial statements should be read in
           conjunction with the consolidated financial statements and the notes
           thereto included in the Company's 1997 Annual Report to Shareholders,
           sections of which are incorporated into the Company's Form 10-K filed
           for the fiscal year ended September 30, 1997. The results of
           operations for the three-month and nine-month periods ended June 30,
           1998 may not necessarily be indicative of the operating results for
           the full year.



(2)        RECLASSIFICATIONS

           Certain reclassifications have been made to prior year amounts in
           order to be consistent with the presentation for the current year.



(3)        INVENTORIES

           Inventories are valued as follows:
           (Dollars in thousands)               June 30,   September 30,
                                                  1998         1997
                                                  ----         ----
           FIFO COST
           Raw materials .................     $ 23,246      $ 17,482
           Work-in-process ...............        3,267         4,483
           Finished goods ................       22,080        13,857
                                               --------      --------
                                                 48,593        35,822
             Excess of FIFO over LIFO cost       (1,259)       (1,225)
                                               --------      --------
                Net Inventories ..........     $ 47,334      $ 34,597
                                               ========      ========



(4)        SHORT-TERM BORROWINGS

           The Company's European subsidiaries have working capital lines of
           credit of approximately $15,000,000. At June 30, 1998 and


                                  Page 6 of 13

<PAGE>   7


             September 30, 1997, $3,383,000 and $3,312,000, respectively, was
             outstanding. At June 30, 1998, interest was at rates ranging from
             4.35% to 8.5%. In addition, a note payable of $2,258,000 and
             $2,008,000 relating to an acquisition was outstanding at June 30,
             1998 and September 30, 1997, respectively. At June 30, 1998, the
             interest rate on the note was 6%.


(5)          LONG-TERM DEBT

             In May, 1998, the Company amended its existing unsecured
             multi-currency revolving loan facility ("Credit Facility"),
             increasing it $50,000,000, bringing the total facility to
             $185,0000. The Credit Facility now matures April 30, 2003 and may
             be extended in one-year increments with the approval of the bank
             group. The Credit Facility includes commitment reductions at
             specified dates and for events throughout the term of the loan,
             however, the commitment does not reduce below $135,000,000.
             Interest rates are based on increments over the LIBOR or foreign
             currency equivalent rate. A 25 basis points facility fee is payable
             on the total amount of the commitment. As of June 30, 1998,
             interest rates ranged from 6.375% to 6.50%. The Company is in
             compliance with all of its covenants under its credit facilities.

             In May 1997, the Company entered into an interest rate swap
             agreement with a commercial bank which effectively converts
             $30,000,000 of its floating rate debt to a fixed rate of 6.33%. The
             effective interest rate on this fixed portion of debt was 7.33% at
             June 30, 1998. The Company does not use derivatives for trading
             purposes.

             Long-term debt consists of the following:
             (In thousands)

                                                March 31,   September 30,
                                                  1998          1997
                                                  ----          ----

               Revolving credit facilities     $ 106,905      $ 79,700
               Other .....................         2,955         2,507
                                                --------       ------- 
                                                 109,860        82,207
               Less current maturities ...        (1,038)         (549)
                                               ---------      --------
               Long-term debt ............     $ 108,822      $ 81,658
                                               =========      ========



(6)          ANTHONY & SYLVAN INITIAL PUBLIC OFFERING AND SPIN-OFF

             On May 8, 1998, the Company announced plans for an initial public
             offering and subsequent tax-free spin-off of its Swimming Pool
             Sales and Installation Segment, into a new company to be known as


                                  Page 7 of 13
<PAGE>   8


              Anthony & Sylvan Pools. The Company plans to file a registration
              statement with the U.S. Securities and Exchange Commission for the
              initial public offering later this calendar year. The tax-free
              spin-off is subject to obtaining a favorable tax ruling from the
              Internal Revenue Service. In order for the spin-off to be tax-free
              at least 80% of the shares of Anthony & Sylvan must be
              distributed. Accordingly, the percentage of shares of Anthony &
              Sylvan to be initially issued will likely be in the range of 15 to
              20 percent. The spin-off is expected to follow the offering by
              several months.


(7)          LITIGATION

             There has been no material change to the status of the litigation
             referred to in the Company's 1997 Annual Report to Shareholders,
             sections of which are incorporated in the Company's Form 10-K filed
             for the fiscal year ended September 30, 1997.


(8)          SUBSEQUENT EVENTS

             On July 8, 1998, the Company completed the sale of its subsidiary
             Enpac Corporation. No material gain or loss will be recognized on
             the sale.

             On July 22, 1998, the Company completed the acquisition of
             substantially all of the operating assets of Rainbow Lifegard,
             Inc., Rainbow Molding, Inc. and Kencar, Inc., collectively known as
             Rainbow for approximately $25,000,000 which was financed under the
             Company's Credit Facility. Rainbow, with revenues of approximately
             $25,000,000 for the year ended January 31, 1998 is a leading
             manufacturer of accessory products for pools, spas and aquariums
             including chlorinators, skimmers, brushes and filters.


















                                  Page 8 of 13
<PAGE>   9


ITEM 2.

     MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
                                  OF OPERATIONS

                 THREE MONTHS ENDED JUNE 30, 1998 COMPARED WITH
                        THREE MONTHS ENDED JUNE 30, 1997

Net sales of $137,625,000 for the third quarter of fiscal 1998 increased 34%
over fiscal 1997 net sales of $102,652,000. The increase in sales was
attributable to the Swimming Pool and Spa Equipment Segment and the Swimming
Pool Sales and Installation Segment, both of which benefited from the
acquisition of General Aquatics on May 1, 1997. This increase was partially
offset by a lower than expected sales increase in the Water Treatment and
Systems Equipment Segment. This resulted from the slowdown in Asian projects,
declining oil prices that have reduced capital available for water treatment
projects and the worldwide over supply of computer chips which has slowed the
demand for ultra-pure water systems in that industry.

Gross profit increased slightly from 29.5% to 30.1% due to improved gross profit
margins in the Swimming Pool and Spa Equipment Segment as a result of
consolidation and reorganization programs implemented subsequent to the
acquisition of General Aquatics on May 1, 1997.

Operating expenses, consisting of engineering and development, selling, and
administrative expenses, as a percentage of sales decreased from 18.3% to 18.2%.
Cost savings attributable to the consolidation and reorganization programs
implemented in the Swimming Pool and Spa Equipment Segment subsequent to the
acquisition of General Aquatics, and lower stock compensation expense due to a
reduction in the number of eligible stock options were offset by the higher cost
structure of the Swimming Pool Sales and Installation Segment, acquired with
General Aquatics on May, 1, 1997.

Interest and other expense increased by $644,000 to $2,287,000. The increase was
the result of increased borrowings, which were used to finance the acquisition
of General Aquatics in the third quarter of fiscal 1997.

The Company's effective tax rate was 35% in both periods.

As a result of the above items, net income of $9,170,000 or $.68 per diluted
share, increased $2,766,000 from $6,404,000 or $.49 per diluted share.





                                  Page 9 of 13

<PAGE>   10


                  NINE MONTHS ENDED JUNE 30, 1998 COMPARED WITH
                         NINE MONTHS ENDED JUNE 30, 1997

Net sales of $308,999,000 increased 56% over fiscal 1997 net sales of
$198,460,000. The increase in sales was primarily attributable to the Swimming
Pool Sales and Installation Segment and the Swimming Pool and Spa Equipment
Segment, both of which benefited from the acquisition of General Aquatics on May
1, 1997. The remaining increase was attributable to a stronger European
contribution to Water Treatment and Systems Equipment Segment, partially offset
by the slowdown in Asian projects, declining oil prices that have reduced
capital available for water treatment projects and the worldwide over supply of
computer chips which has slowed the demand for ultra-pure water systems in that
industry.

Gross Profit decreased from 28.0% of sales to 27.4%. The decrease was entirely
attributable to the inclusion of the Swimming Pool Sales and Installation
Segment which was created upon the acquisition of General Aquatics. The first
and second quarters which are seasonally the slowest for this segment were not
included in last years results, and as such, the inclusion of a full year's
results in fiscal 1998 reduces the overall gross profit margin.

Operating expenses, consisting of engineering and development, selling, and
administrative expenses, as a percentage of sales increased from 18.8% to 19.9%.
The increase is primarily attributable to the inclusion of the off-season first
and second quarters for the Pool Sales and Installation Segment, which was
acquired with General Aquatics on May 1, 1997. This increase was partially
offset by cost savings attributable to the consolidation and reorganization
programs implemented in the Swimming Pool and Spa Equipment Segment, following
the acquisition of General Aquatics, and lower stock compensation expense due to
a reduction in the number of eligible stock options.

Interest and other expense increased by $3,144,000 to $6,052,000. The increase
was the result of increased borrowings, which were used to finance the
acquisition of General Aquatics in the third quarter of fiscal 1997.

The Company's effective tax rate was 35% in both periods.

As a result of the above items, net income of $11,257,000 or $.84 per diluted
share, increased $1,332,000 from $9,925,000 or $.76 per diluted share.

                         LIQUIDITY AND CAPITAL RESOURCES

At June 30, 1998, funded debt was $115,501,000, an increase of $27,974,000 from
September 30, 1997. The increase was primarily attributable to a combination of
capital expenditures and acquisition payments of $12,806,000 and $10,102,000,
respectively, for the first nine months of fiscal 1998. Capital expenditures
relate primarily to

                                  Page 10 of 13

<PAGE>   11

investments made in the Swimming Pool and Spa Equipment Segment plants following
the acquisition of General Aquatics. Acquisition payments were attributable to
add-on acquisitions in the Swimming Pool and Spa Equipment and Swimming Pool
Sales and Installation Segments. Cash used in operating activities for the first
nine months of fiscal 1998 of $3,712,000 includes the normal seasonal working
capital increase.

On May 12, 1998, the Company amended its existing credit agreement. The
amendment included an increase of $50,000,000 in the unsecured multi-currency
revolving facility and a one-year extension of the loan maturity date to April
30, 2003. After the amendment, the total facility is $185,000,000. The Company
believes that funds available under its Credit Facility and funds generated from
operations will be sufficient to satisfy its anticipated operating needs and
capital improvements for the foreseeable future.

The Company is involved in various claims and lawsuits incidental to its
business, including product liability claims which are covered by insurance
after certain deductibles. As discussed in Note 16 of the Consolidated Financial
Statements, included in the Annual Report to Shareholders for the year ended
September 30, 1997, one such lawsuit involves claims against the Company and
other defendents which exceed $200 million, for which management believes it has
meritorious defenses. Although the Company believes that its reserves are
adequate, a significant increase in the aggregate amount of claims could have an
adverse effect on the deductible level or upon the Company's ability to obtain
product liability coverage for certain product lines. While the ultimate result
of these contingencies cannot be predicted with certainty, based on information
presently available, management does not expect these matters to have a material
adverse effect on the consolidated financial position, results of operations or
cash flows of the Company.

                    PRIVATE SECURITIES LITIGATION REFORM ACT

The Private Securities Litigation Reform Act of 1995 provides a "safe harbor"
for forward-looking statements. Certain information included in this report and
other materials filed with the Securites and Exchange Commission (as well as
information included in oral or other written statements made or to be made by
the Company) contains statements that are forward-looking. Such statements may
relate to plans for future expansion, plant integrations and consolidations,
business development activities, other capital spending, financing or the
effects of regulation and competition. Such information involves important risks
and uncertainties that could significantly affect anticipated results in the
future and, accordingly, such results may differ from those expressed in any
forward-looking statements made by or on behalf of the Company. These risks and
uncertainties include, but are not limited to those relating to product
development activities, actual costs of plant integrations and consolidations,
dependence on existing management, global economic and market conditions, the
impact of weather on pool businesses, and changes in federal or state laws.

                                  Page 11 of 13


<PAGE>   12

PART II. OTHER INFORMATION

ITEM 1.  LEGAL PROCEEDINGS

         There has been no material change to the status of the legal
         proceedings referred to in the 1997 Form 10-K during the period
         covered by this report.


ITEM 2.  CHANGES IN SECURITIES

         No change.


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

         None


ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K

         (a)  Exhibits

              10.2                     Amendment No. 2 to Credit Agreement
                                       between Essef Corporation, National City
                                       Bank and ABN Amro Bank N.V. dated as of
                                       May 1, 1998

              11                       Earnings Per Share

              13                       Independent Public Accountants' Review
                                       Report

              15                       Independent Public Accountants'
                                       Awareness Letter

              27                       Financial Data Schedule


         (b)  Form 8-K

              The following reports filed on Form 8-K are incorporated by
              reference.

              Filed 8/5/98
              ------------

              Asset Purchase Agreement among Rainbow Acquisition Corporation,
              The Price Family Trust UAD 11697, Price-Presby Companies, Inc.,
              Rainbow Molding, Inc., Rainbow Lifeguard, Inc. and Kencar, Inc
              dated June 1, 1998.

                                  Page 12 of 13


<PAGE>   13



SIGNATURE

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.



                                           ESSEF Corporation
                                           (Registrant)




                                           Thomas B. Waldin
                                           -----------------------------
                                           THOMAS B. WALDIN
                                           President and
                                           Chief Executive Officer
                                           (Principal Executive Officer)




                                           Stuart D. Neidus
                                           -----------------------------
                                           STUART D. NEIDUS
                                           Executive Vice President
                                           and Chief Financial Officer
                                           (Principal Accounting Officer)






Date: August 10, 1998










                                  Page 13 of 13