<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                              Washington, DC 20549

                                   Form 10-Q/A
                                 Amendment No. 1

                   QUARTERLY REPORT UNDER SECTION 13 OR 15 (d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934


                        For Quarter Ended April 30, 1997

                          Commission File Number 1-9579


                                   Ecogen Inc.
             (Exact name of registrant as specified in its charter)



              Delaware                                        22-2487948
(State or other jurisdiction of                           (I.R.S. Employer
 incorporation or organization)                           Identification Number)


      2005 Cabot Boulevard West, Langhorne, Pennsylvania         19047
     (Address of principal executive offices)                  (Zip Code)



Registrant's telephone number,
including area code                                            (2l5) 757-l590


     Indicate by check mark whether the registrant (l) has filed all reports
required to be filed by Section l3 or l5 (d) of the Securities Exchange Act of
l934 during the preceding l2 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    .
                                              ---     ---

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

<TABLE>
<CAPTION>
             Class                                 Outstanding at June 1, l997
             -----                                 ---------------------------
<S>                                                <C>
Common Stock, $.01 par value                                  7,992,072
</TABLE>


                                       1
<PAGE>   2
                AMENDMENT NO. 1 TO QUARTERLY REPORT ON FORM 10-Q
                   FOR THE FISCAL QUARTER ENDED APRIL 30, 1997

       The undersigned registrant hereby amends ITEM 6(a) of its Quarterly
Report on Form 10-Q for the fiscal quarter ended April 30, 1997 to read in its
entirety as set forth below:

   
ITEM 6(a):     Exhibits

Exhibit No.    Description

3.1            Restated Certificate of Incorporation of Ecogen Inc. (Form 10-Q
               for fiscal quarter ended January 31, 1996.)*

3.8            By-Laws of Ecogen Inc., as amended. (Form S-1 Registration
               Statement.)*

10.125         Amendment No. 1 to Research and Development Agreement by and
               between Monsanto Company and Ecogen Inc. dated May 22, 1997.

27             Financial Data Schedule
    

------------------
*  These items are hereby incorporated by reference from the exhibits of the
   filing or report indicated (except where noted, Commission File No. 1-9579,
   and File No. 33-40319 in the case of the Form S-1 Registration Statement).



                                       2
<PAGE>   3
                                   Signatures


       Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this Amendment No. 1 to be signed on its behalf by
the undersigned thereunto duly authorized.

   
Date:  July 29, 1997
    

                                ECOGEN INC.



                                By:  /s/ JAMES P. REILLY, JR.
                                     ------------------------------------------
                                     James P. Reilly, Jr.
                                     Chairman and Chief Executive Officer



                                By:  /s/ MARY E. PAETZOLD
                                     ------------------------------------------
                                     Mary E. Paetzold
                                     Vice President and Chief Financial Officer


                                       3