<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       December 31, 1997
                                        -----------------------

+--+
|  |     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 February 9, 1998   
-------------------------------           --------------------------------
Common Shares, no par value                       10,610,958 Shares


                                  Page 1 of 16

<PAGE>   2
                              ESSEF CORPORATION
                                  FORM 10-Q

                       For Quarter Ended December 31, 1997

                                      INDEX
<TABLE>
<CAPTION>

                                                               Sequential
                                                                Page No.
                                                               ----------

<S>  <C>                                                            <C>
Part I - Financial Information
     Item 1.  Financial Statements
          Condensed Consolidated Balance Sheets -
            December 31, 1997 and September 30, 1997 ...............    3
          Condensed Consolidated Statements of Income -
            Three Months Ended December 31, 1997 and 1996 ..........    4
          Condensed Consolidated Statements of Cash Flows -
            Three Months Ended December 31, 1997 and 1996 ..........    5
     Notes to Condensed Consolidated Financial
            Statements .............................................  6-8

     Item 2.  Management's Discussion and Analysis of
              Financial Condition and Results of Operations ........ 9-10

Part II - Other Information

     Item 1.  Legal Proceedings ....................................   11

     Item 2.  Changes in Securities ................................   11

     Item 4.  Submission of Matters to a Vote of Security
              Holders ..............................................   11

     Item 6.  Exhibits and Reports on Form 8-K .....................11-16

</TABLE>


                                  Page 2 of 16
<PAGE>   3

PART I.  FINANCIAL INFORMATION

ITEM 1.  FINANCIAL STATEMENTS
                   ESSEF CORPORATION AND SUBSIDIARIES
                   CONDENSED CONSOLIDATED BALANCE SHEETS
                             (DOLLARS IN THOUSANDS)
<TABLE>
<CAPTION>
                                                                  December 31,   September 30,
                                                                     1997            1997
                                                                   ---------       ---------
ASSETS                                                            (unaudited)      (audited)
------         

<S>                                                                <C>             <C>      
Current Assets
    Cash and cash equivalents ...............................      $     428       $   1,668
    Accounts receivable, net ................................         42,144          39,512
    Inventories, net ........................................         38,035          34,597
    Prepayments and other ...................................          2,780           2,173
                                                                   ---------       ---------
       Total current assets .................................         83,387          77,950

Property, plant and equipment, net ..........................         65,977          63,820
Real estate held for sale ...................................          4,333           4,333
Goodwill, net ...............................................         60,538          60,349
Deferred income taxes .......................................          5,392           5,706
Other .......................................................          5,110           4,725
                                                                   ---------       ---------
                                                                   $ 224,737       $ 216,883
                                                                   =========       =========

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

Current Liabilities
    Short-term borrowings ...................................      $   3,600       $   5,320
    Current maturities of long-term debt ....................            605             549
    Accounts payable ........................................         23,785          20,028
    Accrued expenses ........................................         20,625          29,237
    Accrued income taxes ....................................          6,465           8,668
                                                                   ---------       ---------
       Total current liabilities ............................         55,080          63,802

Long-Term Debt ..............................................         99,480          81,658
Other Long-Term Liabilities .................................          5,203           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
       11,105,516 and 11,090,431 shares,
       respectively .........................................         31,357          32,234
       503,927 Treasury shares at cost ......................         (7,962)         (7,962)
    Retained earnings .......................................         41,181          41,099
    Foreign currency translation adjustment .................            398              75
                                                                   ---------       ---------
           Total shareholders' equity .......................         64,974          65,446
                                                                   ---------       ---------
                                                                   $ 224,737       $ 216,883
                                                                   =========       =========
</TABLE>

See notes to condensed consolidated financial statements.

                                  Page 3 of 16
<PAGE>   4

                       ESSEF CORPORATION AND SUBSIDIARIES
                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME

                      (IN THOUSANDS, EXCEPT PER SHARE DATA)
                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                               Three Months Ended
                                                   December 31,
                                           --------------------------
                                               1997            1996
                                           ----------      ----------

<S>                                        <C>             <C>       
Net sales ...........................      $   79,618      $   40,209

Cost of sales .......................          61,487          30,094
                                           ----------      ----------
         Gross profit ...............          18,131          10,115

Operating expenses ..................          16,397           8,475
                                           ----------      ----------
     Income from operations .........           1,734           1,640

Interest and other expense ..........           1,608             591
                                           ----------      ----------
         Income before income taxes .             126           1,049

Provision for income taxes ..........              44             367
                                           ----------      ----------

         Net income .................      $       82      $      682
                                           ==========      ==========


Earnings Per Share:

         Basic (a) ..................      $      .01      $      .06
         Diluted (a) ................      $      .01      $      .06


Average shares outstanding:

         Basic ......................          10,591          10,566
         Diluted ....................          12,106          11,852

</TABLE>


(a)      Basic and diluted earnings per share have been calculated in
         accordance with SFAS No. 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 16
<PAGE>   5


                       ESSEF CORPORATION AND SUBSIDIARIES
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS

                                 (IN THOUSANDS)
                                   (UNAUDITED)
<TABLE>
<CAPTION>
                                                              Three Months Ended     
                                                                  December 31,       
                                                           ------------------------ 
                                                               1997            1996   
                                                           --------       --------
                                                           
<S>                                                        <C>            <C>     
Cash Flows from Operating Activities
      Net income ....................................      $     82       $    682
      Adjustments to reconcile net income
      to net cash used in operating activities
                Depreciation and amortization .......         2,842          1,199
                Other ...............................           262            193
   Change in operating assets and liabilities
           Accounts receivable ......................        (2,568)          (663)
           Inventories ..............................        (3,372)        (3,810)
           Prepayments and other assets .............          (562)          (245)
           Accounts payable .........................         3,770          1,224
           Accrued expenses .........................       (11,347)        (4,117)
           Accrued and deferred income taxes ........        (1,931)           360
                                                           --------       --------
                Net cash used in operating activities       (12,824)        (5,177)
                                                           --------       --------

Cash Flows from Investing Activities
      Additions to property, plant and
           equipment ................................        (4,701)        (2,121)
      Business acquisitions .........................          (583)        (1,210)
      Other, net ....................................           634            129
                                                           --------       --------
                Net cash used in investing activities        (4,650)        (3,202)
                                                           --------       --------

Cash Flows from Financing Activities
      Proceeds from long term debt ..................        17,878          7,566
      (Decrease) in short-term borrowings ...........        (1,720)          (733)
      Proceeds from exercise of stock options .......            76           --
                                                           --------       --------
           Net cash provided by financing activities         16,234          6,833
                                                           --------       --------

Net decrease in cash and cash equivalents ...........        (1,240)        (1,546)

Cash and Cash Equivalents
  Beginning of period ...............................         1,668          2,620
                                                           --------       --------
  End of period .....................................      $    428       $  1,074
                                                           ========       ========


Supplemental Cash Flow Information
           Interest paid ............................      $  1,393       $    548
                                                           ========       ========
           Income taxes paid ........................      $  1,928       $   --
                                                           ========       ========
</TABLE>


See notes to condensed consolidated financial statements.


                                  Page 5 of 16

<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 December 31,
           1997, and the results of their operations and their cash flows for
           the three month periods ended December 31, 1997 and 1996.

           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 period ended December 31, 1997 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:
<TABLE>
<CAPTION>
           (Dollars in thousands)                  December 31,  September 30,
                                                      1997           1997                   
                                                    --------       --------  
           FIFO COST
<S>                                                 <C>            <C>       
               Raw materials .................      $ 16,197       $ 11,932  
               Work-in-process ...............         6,342         10,033  
               Finished goods ................        16,893         13,857  
                                                    --------       --------  
                                                      39,432         35,822  
                 Excess of FIFO over LIFO cost        (1,397)        (1,225) 
                                                    --------       --------  
                    Net Inventories ..........      $ 38,035       $ 34,597  
                                                    ========       ========  
</TABLE>
               

(4)        SHORT-TERM BORROWINGS

           The Company's European subsidiaries have working capital lines of
           credit of approximately $15,000,000. At December 31, 1997 and
           September 30, 1997, $1,592,000 and $3,312,000, respectively was
           outstanding. At December 31, 1997 interest was at rates ranging from
           4.35% to 9.0%. In addition, a note payable of $2,008,000 relating to
           an acquisition was outstanding at December 31, 1997. The interest
           rate on the note is 6%.

                                  Page 6 of 16
<PAGE>   7


(5)        LONG-TERM DEBT

           The Company through its bank group has an unsecured $135,000,000
           multi-currency revolving loan facility ("Credit Facility"). The
           Credit Facility matures April 30, 2002 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 $100,000,000. Interest rates are based on increments
           over LIBOR (or foreign currency equivalent) rate. A 20 basis points
           facility fee is payable on the total amount of the commitment. The
           Company is in compliance with all of its covenants under its credit
           facilities. As of December 31, 1997, interest rates ranged from 6.16%
           to 6.33%.

           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 6.96% at December 31, 1997.
           The Company does not use derivatives for trading purposes.

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

<TABLE>
<CAPTION>
                                              December 31,   September 30,
                                                 1997             1997
                                               ---------       ---------   
                                              (unaudited)       (audited)

<S>                                            <C>             <C>         
           Revolving credit facilities         $  97,450       $  79,700   
           Other                                   2,635           2,507   
                                               ---------       ---------   
                                                 100,085          82,207   
           Less current maturities                  (605)           (549)  
                                               ---------       ---------   
           Long-term debt                      $  99,480       $  81,658   
                                               =========       =========   
                                                                             
</TABLE>
             

(6)        ACCOUNTING FOR SFAS NO. 128 "EARNINGS PER SHARE"

           The Financial Accounting Standards Board recently issued SFAS No.
           128, "Earnings Per Share," which requires adoption for financial
           statements issued for periods ending after December 15, 1997.
           Accordingly, the Company adopted this new standard effective October
           1, 1997. Prior period results have been restated to reflect the
           adoption of the new standard.


(7)        SUBSEQUENT EVENTS

           On January 29, 1998, the Board of Directors authorized a 10% stock
           dividend to be distributed on or about March 3, 1998 to

                                  Page 7 of 16

<PAGE>   8

           shareholders of record on February 12, 1998. The consolidated
           financial statements have not been restated to reflect the number of
           shares outstanding following the dividend.

(8)        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.


                                  Page 8 of 16


<PAGE>   9


ITEM 2.

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

               THREE MONTHS ENDED DECEMBER 31, 1997 COMPARED WITH
                      THREE MONTHS ENDED DECEMBER 31, 1996

Net sales in 1997 of $79,618,000 increased 98% from 1996 net sales of
$40,209,000 for the first quarter. The Swimming Pool Sales and Installation
Segment, and the Swimming Pool and Spa Equipment Segment, which benefited from
the acquisition of General Aquatics on May 1, 1997 together accounted for over
90% of the increase. The Water Treatment and Systems Equipment Segment posted a
10% increase in sales.

Gross Profit decreased from 25.2% to 22.8% of sales for the first quarter which
was primarily attributable to the impact of the General Aquatics acquisition on
the Swimming Pool and Spa Equipment Segment. The General Aquatics acquisition
increased the seasonality of the Company's overall sales, with the first quarter
being a seasonally slow quarter for this segment. Gross margins in this segment
were also impacted by costs associated with inefficiencies related to the
start-up of operations following the previously announced plant closing at City
of Industry, California.

Operating expenses, consisting of engineering and development, selling, and
administrative expenses, as a percentage of sales decreased from 21.1% to 20.6%
which was mainly attributable to an adjustment in the accrual for incentive
based employee stock options due to a reduction in the number of eligible stock
options.

Interest and other expense increased by $1,017,000 to $1,608,000. 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 $82,000, or $.01 per diluted
share, decreased $600,000 from $682,000 or $.06 per diluted share.


                         LIQUIDITY AND CAPITAL RESOURCES

At December 31, 1997 funded debt was $103,685,000 an increase of $16,158,000
from September 30, 1997. The increase came principally as a result of normal
seasonal working capital requirements which resulted in a working capital
increase from September 30, 1997 of $14,159,000 to $28,307,000 at December 31,
1997.


                                  Page 9 of 16
<PAGE>   10


Also impacting the Company's debt were capital expenditures for the first
quarter which totaled $4,701,000 compared to $2,121,000 for the same period last
year. The increase in capital expenditures relates primarily to continued
investments made in India as part of the global expansion strategy and
investments made in the Company's Swimming Pool and Spa Equipment Segment plants
following the acquisition of General Aquatics which resulted in the closure of
the City of Industry plant and consolidation of its operations into the
facilities at Moorpark, California and Sanford, North Carolina.

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 in fiscal 1998.

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 Securities 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 10 of 16

<PAGE>   11
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

         The Annual Meeting of Shareholders of the Company (the "Annual
         Meeting") was held on January 29, 1998. Of the 10,592,368 shares of
         Common Stock outstanding and entitled to vote at the Annual Meeting,
         10,094,900 shares were each present in person or by proxy, each
         entitled to one vote on each matter to come before the meeting.

         The following matter was submitted to a vote of security holders of the
         Company at the Annual Meeting, with the results indicated below:

         Proposal to approve amendment in the Company's Articles of
         Incorporation to effect a change in the number of common shares
         authorized to be issued and outstanding from 15,000,000 to 40,000,000:

<TABLE>

<S>                                                          <C>      
         Votes cast FOR the proposal:                        9,210,886
         Votes cast AGAINST the proposal:                      874,230
         Votes WITHHELD:                                         9,784

         Shares held by brokers and nominees:                7,583,990
         Shares held by brokers and nominess not voted:        349,810
</TABLE>


ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K

         (a)  Exhibits

              11         Earnings Per Share

              13         Independent Public Accountants' Review Report


                                  Page 11 of 16



<PAGE>   12




              15         Independent Public Accountants' Awareness Letter

              27         Financial Data Schedule


        (b)  Form 8-K

         No reports on Form 8-K have been filed during the quarter for which
         this Report is filed.
















                                  Page 12 of 16


<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: February 9, 1998












                                  Page 13 of 16