<PAGE>   1
      As filed with the Securities and Exchange Commission on June 10, 1999


                                                    Registration No. 33-

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

                                    FORM S-8
             REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

                                   ECOGEN INC.
             (Exact name of registrant as specified in its charter)

                      DELAWARE                             22-2487948
         (State or other jurisdiction of       (IRS Employer Identification No.)
          incorporation or organization)

         2000 CABOT BOULEVARD WEST, SUITE 170                 19047
              LANGHORNE, PENNSYLVANIA                       (Zip Code)
      (Address of Principal Executive Offices)

                                   ECOGEN INC.
                             1999 STOCK OPTION PLAN
                            (Full title of the plan)

             MARY E. PAETZOLD                          WITH A COPY TO:
VICE PRESIDENT AND CHIEF FINANCIAL OFFICER        ELIZABETH A. BROWER, ESQ.
                ECOGEN INC.                PAUL, HASTINGS, JANOFSKY & WALKER LLP
 2000 CABOT BOULEVARD WEST, SUITE 170             1055 WASHINGTON BOULEVARD
       LANGHORNE, PENNSYLVANIA 19047            STAMFORD, CONNECTICUT  06901
 (Name and address of agent for service)               (203) 961-7400

            (215) 757-1590
 (Telephone number, including area code,
         of agent for service)

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

                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
===========================================================================================================
                                             PROPOSED             PROPOSED
TITLE OF                                     MAXIMUM              MAXIMUM
SECURITIES              AMOUNT               OFFERING             AGGREGATE               AMOUNT OF
TO BE                   TO BE                PRICE                OFFERING                REGISTRATION
REGISTERED              REGISTERED(1)        PER SHARE(2)         PRICE                   FEE
-----------------------------------------------------------------------------------------------------------
<S>                      <C>                   <C>                <C>                      <C>
Common Stock,            1,500,000             $3.2813            $4,921,950               $1,368.30
Par Value $.01
===========================================================================================================
</TABLE>

(1)   Pursuant to Rule 416(b) of the Securities Act of 1933, as amended (the
      "Securities Act"), this amount shall be deemed to cover securities
      resulting from the split of, or the stock dividend on, or any other
      anti-dilution mechanisms on, the registered securities.

(2)   Estimated solely for the purpose of calculating the registration fee in
      accordance with Rule 457(c), based on the average of the high and low
      prices ($3.2813 per share) of our Common Stock on the NASDAQ National
      Market System on June 7, 1999.


================================================================================

<PAGE>   2


                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

ITEM 1.  PLAN INFORMATION.*

ITEM 2.  REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION.*

         * Pursuant to the Note to Part I of Form S-8, the information required
by Part I is not being filed with the Securities and Exchange Commission.


                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         In accordance with General Instruction E of Form S-8 regarding the
registration of additional securities of the same class as other securities for
which a previously filed registration statement on Form S-8 is effective, (i)
the contents of our previously filed Registration Statement on Form S-8 and Form
S-3 (No. 33-23767) filed on August 24, 1988, which registered shares of our
common stock, par value $.01 per share (the "Common Stock"), acquired or to be
acquired pursuant to our 1986 Employee Stock Purchase Plan and shares of our
Common Stock underlying options granted or to be granted pursuant to our Stock
Option Plan (the "Existing Option Plan"), (ii) the contents of the previously
filed Registration Statement on Form S-8 (No. 33-39687) filed on April 1, 1991,
which registered additional shares of our Common Stock underlying options
granted or to be granted pursuant to the Existing Option Plan, (iii) the
contents of the previously filed Registration Statement on Form S-8 (No.
33-50478) filed on August 5, 1992, which registered additional shares of our
Common Stock underlying options granted or to be granted pursuant to the
Existing Option Plan and (iv) the contents of the previously filed Registration
Statement on Form S-8 (No. 33-70538) filed on October 20, 1993, which registered
additional shares of our Common Stock underlying options granted or to be
granted pursuant to the Existing Option Plan, are incorporated herein by
reference.

ITEM 3.  INCORPORATION OF DOCUMENTS BY REFERENCE.

         (a) Our Annual Report on Form 10-K for the fiscal year ended October
31, 1998, filed on January 15, 1999 pursuant to Section 13 or 15(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), as amended
through the date hereof.

         (b) Our Quarterly Reports on Form 10-Q for the quarters ended (i)
January 31, 1999, filed on March 17, 1999 and (ii) April 30, 1999, filed on June
10, 1999, pursuant to Section 13 or 15(d) of the Exchange Act.

         (c) The description of our Common Stock contained in our Registration
Statement on Form 8-A (File No. 1-95791) filed on June 12, 1987 under Section
12(g) of the Exchange Act, including any amendments or reports filed for the
purpose of updating such description.

         (d) All reports and other documents subsequently filed by us pursuant
to Section 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing
of a post-effective amendment which indicates that all securities offered hereby
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference herein and to be part hereof from the
date of the filing of such reports and documents.

ITEM 4.  DESCRIPTION OF SECURITIES.

         Not applicable.


<PAGE>   3


ITEM 5.   INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Our counsel, Paul, Hastings, Janofsky & Walker LLP, 1055 Washington
Boulevard, Stamford, Connecticut 06901, has rendered an opinion to us to the
effect that the shares of our Common Stock registered hereby have been duly
authorized and will be, upon original issuance pursuant to the terms of our 1999
Stock Option Plan, legally and validly issued, fully paid and non-assessable.
Esteban A. Ferrer, a member of Paul, Hastings, Janofsky & Walker LLP, is one of
our directors.

ITEM 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Article VIII of our Bylaws, as amended, provides generally for
indemnification of our officers, directors, agents and employees to the extent
authorized by the General Corporation Law of the State of Delaware. Pursuant to
Section 145 of the Delaware General Corporation Law, a corporation generally has
the power to indemnify its present and former directors, officers, employees and
agents against expenses incurred by them in connection with any suit to which
they are, or are threatened to be made, a party by reason of their serving in
such positions so long as they acted in good faith and in a manner they
reasonably believed to be in, or not opposed to, the best interests of a
corporation, and with respect to any criminal action, they had no reasonable
cause to believe their conduct was unlawful. With respect to suits by or in the
right of a corporation, however, indemnification is not available if such person
is adjudged to be liable for negligence or misconduct in the performance of his
duty to the corporation unless the court determines that indemnification is
appropriate. In addition, a corporation has the power to purchase and maintain
insurance for such persons. We currently maintain such directors' and officers'
insurance. The statute also expressly provides that the power to indemnify
authorized thereby is not exclusive of any rights granted under any bylaw,
agreement, vote of stockholders or disinterested directors, or otherwise.

         As permitted by Section 102 of the Delaware General Corporation Law,
our stockholders have approved and incorporated provisions into our Restated
Certificate of Incorporation eliminating a director's personal liability for
monetary damages to us and our stockholders arising from a breach of a
director's fiduciary duty, except for liability under Section 174 of the
Delaware General Corporation Law or liability for any breach of the director's
duty of loyalty to us or our stockholders, for acts or omissions not in good
faith or that involve intentional misconduct or a knowing violation of law or
for any transaction in which the director derived an improper personal benefit.

         We have entered into indemnification agreements with our directors and
officers. These agreements provide broader indemnity rights than those provided
under the Delaware General Corporation Law and the Company's Bylaws. The
indemnification agreements are not intended to deny or otherwise limit third
party or derivative suits against the Company or its directors or officers, but
to the extent a director or officer were entitled to indemnity or contribution
under the indemnification agreement, the financial burden of a third party suit
would be borne by the Company, and the Company would not benefit from derivative
recoveries against the director or officer. Such recoveries would accrue to the
benefit of the Company but would be offset by the Company's obligations to the
director or officer under the indemnification agreement.

         The above discussion of the Company's Bylaws, Restated Certificate of
Incorporation and indemnification agreements and of Section 145 of the Delaware
General Corporation Law is not intended to be exhaustive and is qualified in its
entirety by such Bylaws, Restated Certificate of Incorporation, indemnification
agreements and statute.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8.  EXHIBITS.

         The exhibits filed as part of this Registration Statement are as
follows:


                                      -2-
<PAGE>   4

     EXHIBIT NO.  DESCRIPTION
     -----------  -----------

         4.1      Restated Certificate of Incorporation of Ecogen Inc. (Form
                  10-K for the fiscal year ended December 31, 1992)*

         4.2      Bylaws, as amended (Form S-1, Registration Statement, File No.
                  33-14119)*

         4.3      1999 Stock Option Plan (Form 10-Q for the fiscal quarter ended
                  April 30, 1999, filed on June 10, 1999)*

         5        Opinion of Paul, Hastings, Janofsky & Walker LLP

        23.1      Consent of KPMG LLP

        23.2      Consent of Paul, Hastings, Janofsky & Walker LLP (contained in
                  Exhibit 5)

        24.1      Power of Attorney (contained on the Signature Page of this
                  Registration Statement)

*        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 are hereby made a part of this Registration Statement.

ITEM 9.  UNDERTAKINGS.

         (a) We hereby undertake:

                  (1) That, for the purpose of determining any liability under
the Securities Act, each post-effective amendment to this Registration Statement
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial bona fide offering thereof.

                  (2) To remove from registration by means of a post-effective
amendment to this Registration Statement any of the securities being registered
which remain unsold at the termination of the offering.

         (b) We hereby undertake that, for purposes of determining any liability
under the Securities Act, each filing of our annual report pursuant to Section
13(a) or Section 15(d) of the Exchange Act that is incorporated by reference in
the Registration Statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

         (c) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to our directors, officers and controlling
persons pursuant to the foregoing provisions or otherwise, we have been advised
that in the opinion of the Commission such indemnification is against public
policy as expressed in the Securities Act and is, therefore, unenforceable. In
the event that a claim for indemnification against such liabilities (other than
the payment by us of expenses incurred or paid by our director, officer or
controlling person in the successful defense of any action, suit or proceeding)
is asserted by such director, officer or controlling person in connection with
the securities being registered, we will, unless in the opinion of our counsel
the matter has been settled by controlling precedent, submit to a court of
appropriate jurisdiction the question whether such indemnification by it is
against public policy as expressed in the Securities Act and will be governed by
the final adjudication of such issue.


                                      -3-
<PAGE>   5

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, we certify that we
have reasonable grounds to believe that we meet all of the requirements for
filing on Form S-8 and have duly caused this Registration Statement to be signed
on our behalf by the undersigned, thereunto duly authorized, in Langhorne,
Commonwealth of Pennsylvania, on June 10, 1999.

                                   ECOGEN INC.


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


                               POWER OF ATTORNEY

         KNOW ALL MEN BY THESE PRESENTS, that each person whose signature
appears below constitutes and appoints James P. Reilly, Jr. and Mary E.
Paetzold, or either of them, his attorney-in-fact, each with the power of
substitution, for him in any and all capacities, to sign any amendments to this
Registration Statement, and to file the same, with exhibits thereto and other
documents in connection therewith, with the Securities and Exchange Commission,
hereby ratifying and confirming all that each of said attorneys-in-fact, or his
substitute or substitutes, may do or cause to be done by virtue thereof.

         Pursuant to the requirements of the Securities Act, this Registration
Statement has been signed by the following persons in the capacities and on the
dates indicated.

<TABLE>
<CAPTION>
Signature                                   Title                               Date
---------                                   -----                               ----
<S>                                 <C>                                         <C>

/s/ James P. Reilly, Jr.            Chairman, Chief Executive Officer           June 10, 1999
------------------------            and Director (Principal Executive
James P. Reilly, Jr.                Officer)

/s/ Mary E. Paetzold
------------------------            Vice President and                          June 10, 1999
Mary E. Paetzold                    Chief Financial Officer
                                    (Principal Financial Officer
                                    and Principal Accounting Officer)

/s/ Esteban A. Ferrer
------------------------            Director                                    June 10, 1999
Esteban A. Ferrer

/s/ Philippe D. Katz
------------------------            Director                                    June 10, 1999
Philippe D. Katz

/s/ Lowell N. Lewis
------------------------            Director                                    June 10, 1999
Lowell N. Lewis

/s/ John R. Sutley
------------------------            Director                                    June 9, 1999
John R. Sutley
</TABLE>



                                      -4-


<PAGE>   6




                                  EXHIBIT INDEX

     EXHIBIT NO.  DESCRIPTION
     -----------  -----------

          5       Opinion of Paul, Hastings, Janofsky & Walker LLP

         23.1     Consent of KPMG LLP

         23.2     Consent of Paul, Hastings, Janofsky & Walker LLP (contained in
                  Exhibit 5)

         24.1     Power of Attorney (contained on the Signature Page of this
                  Registration Statement)


                                      -5-
[{"type": "S-8", "sequence": "1", "description": "FORM S-8 ECOGEN INC.", "secsgml_size_bytes": 17667}]
{"company-data": {"conformed-name": "ECOGEN INC", "cik": "0000814050", "assigned-sic": "2870", "irs-number": "222487948", "state-of-incorporation": "DE", "fiscal-year-end": "1031"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-80405", "film-number": "99644208"}, "business-address": {"street1": "2005 CABOT BLVD W", "city": "LANGHORNE", "state": "PA", "zip": "19047-1810", "phone": "2157571590"}, "mail-address": {"street1": "2005 CABOT BLVD W", "city": "LANGHORNE", "state": "PA", "zip": "19047"}}
1
      As filed with the Securities and Exchange Commission on June 10, 1999

                                                    Registration No. 33-

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

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

                                    FORM S-8
             REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

                                   ECOGEN INC.
             (Exact name of registrant as specified in its charter)

                      DELAWARE                             22-2487948
         (State or other jurisdiction of       (IRS Employer Identification No.)
          incorporation or organization)

         2000 CABOT BOULEVARD WEST, SUITE 170                 19047
              LANGHORNE, PENNSYLVANIA                       (Zip Code)
      (Address of Principal Executive Offices)

                                   ECOGEN INC.
                             1999 STOCK OPTION PLAN
                            (Full title of the plan)

             MARY E. PAETZOLD                          WITH A COPY TO:
VICE PRESIDENT AND CHIEF FINANCIAL OFFICER        ELIZABETH A. BROWER, ESQ.
                ECOGEN INC.                PAUL, HASTINGS, JANOFSKY & WALKER LLP
 2000 CABOT BOULEVARD WEST, SUITE 170             1055 WASHINGTON BOULEVARD
       LANGHORNE, PENNSYLVANIA 19047            STAMFORD, CONNECTICUT  06901
 (Name and address of agent for service)               (203) 961-7400

            (215) 757-1590
 (Telephone number, including area code,
         of agent for service)

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

                         CALCULATION OF REGISTRATION FEE
===========================================================================================================
                                             PROPOSED             PROPOSED
TITLE OF                                     MAXIMUM              MAXIMUM
SECURITIES              AMOUNT               OFFERING             AGGREGATE               AMOUNT OF
TO BE                   TO BE                PRICE                OFFERING                REGISTRATION
REGISTERED              REGISTERED(1)        PER SHARE(2)         PRICE                   FEE
-----------------------------------------------------------------------------------------------------------
Common Stock,            1,500,000             $3.2813            $4,921,950               $1,368.30
Par Value $.01
===========================================================================================================
(1)   Pursuant to Rule 416(b) of the Securities Act of 1933, as amended (the
      "Securities Act"), this amount shall be deemed to cover securities
      resulting from the split of, or the stock dividend on, or any other
      anti-dilution mechanisms on, the registered securities.

(2)   Estimated solely for the purpose of calculating the registration fee in
      accordance with Rule 457(c), based on the average of the high and low
      prices ($3.2813 per share) of our Common Stock on the NASDAQ National
      Market System on June 7, 1999.

================================================================================
2

                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS

ITEM 1.  PLAN INFORMATION.*

ITEM 2.  REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION.*

         * Pursuant to the Note to Part I of Form S-8, the information required
by Part I is not being filed with the Securities and Exchange Commission.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         In accordance with General Instruction E of Form S-8 regarding the
registration of additional securities of the same class as other securities for
which a previously filed registration statement on Form S-8 is effective, (i)
the contents of our previously filed Registration Statement on Form S-8 and Form
S-3 (No. 33-23767) filed on August 24, 1988, which registered shares of our
common stock, par value $.01 per share (the "Common Stock"), acquired or to be
acquired pursuant to our 1986 Employee Stock Purchase Plan and shares of our
Common Stock underlying options granted or to be granted pursuant to our Stock
Option Plan (the "Existing Option Plan"), (ii) the contents of the previously
filed Registration Statement on Form S-8 (No. 33-39687) filed on April 1, 1991,
which registered additional shares of our Common Stock underlying options
granted or to be granted pursuant to the Existing Option Plan, (iii) the
contents of the previously filed Registration Statement on Form S-8 (No.
33-50478) filed on August 5, 1992, which registered additional shares of our
Common Stock underlying options granted or to be granted pursuant to the
Existing Option Plan and (iv) the contents of the previously filed Registration
Statement on Form S-8 (No. 33-70538) filed on October 20, 1993, which registered
additional shares of our Common Stock underlying options granted or to be
granted pursuant to the Existing Option Plan, are incorporated herein by
reference.

ITEM 3.  INCORPORATION OF DOCUMENTS BY REFERENCE.

         (a) Our Annual Report on Form 10-K for the fiscal year ended October
31, 1998, filed on January 15, 1999 pursuant to Section 13 or 15(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), as amended
through the date hereof.

         (b) Our Quarterly Reports on Form 10-Q for the quarters ended (i)
January 31, 1999, filed on March 17, 1999 and (ii) April 30, 1999, filed on June
10, 1999, pursuant to Section 13 or 15(d) of the Exchange Act.

         (c) The description of our Common Stock contained in our Registration
Statement on Form 8-A (File No. 1-95791) filed on June 12, 1987 under Section
12(g) of the Exchange Act, including any amendments or reports filed for the
purpose of updating such description.

         (d) All reports and other documents subsequently filed by us pursuant
to Section 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing
of a post-effective amendment which indicates that all securities offered hereby
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference herein and to be part hereof from the
date of the filing of such reports and documents.

ITEM 4.  DESCRIPTION OF SECURITIES.

         Not applicable.
3

ITEM 5.   INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Our counsel, Paul, Hastings, Janofsky & Walker LLP, 1055 Washington
Boulevard, Stamford, Connecticut 06901, has rendered an opinion to us to the
effect that the shares of our Common Stock registered hereby have been duly
authorized and will be, upon original issuance pursuant to the terms of our 1999
Stock Option Plan, legally and validly issued, fully paid and non-assessable.
Esteban A. Ferrer, a member of Paul, Hastings, Janofsky & Walker LLP, is one of
our directors.

ITEM 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Article VIII of our Bylaws, as amended, provides generally for
indemnification of our officers, directors, agents and employees to the extent
authorized by the General Corporation Law of the State of Delaware. Pursuant to
Section 145 of the Delaware General Corporation Law, a corporation generally has
the power to indemnify its present and former directors, officers, employees and
agents against expenses incurred by them in connection with any suit to which
they are, or are threatened to be made, a party by reason of their serving in
such positions so long as they acted in good faith and in a manner they
reasonably believed to be in, or not opposed to, the best interests of a
corporation, and with respect to any criminal action, they had no reasonable
cause to believe their conduct was unlawful. With respect to suits by or in the
right of a corporation, however, indemnification is not available if such person
is adjudged to be liable for negligence or misconduct in the performance of his
duty to the corporation unless the court determines that indemnification is
appropriate. In addition, a corporation has the power to purchase and maintain
insurance for such persons. We currently maintain such directors' and officers'
insurance. The statute also expressly provides that the power to indemnify
authorized thereby is not exclusive of any rights granted under any bylaw,
agreement, vote of stockholders or disinterested directors, or otherwise.

         As permitted by Section 102 of the Delaware General Corporation Law,
our stockholders have approved and incorporated provisions into our Restated
Certificate of Incorporation eliminating a director's personal liability for
monetary damages to us and our stockholders arising from a breach of a
director's fiduciary duty, except for liability under Section 174 of the
Delaware General Corporation Law or liability for any breach of the director's
duty of loyalty to us or our stockholders, for acts or omissions not in good
faith or that involve intentional misconduct or a knowing violation of law or
for any transaction in which the director derived an improper personal benefit.

         We have entered into indemnification agreements with our directors and
officers. These agreements provide broader indemnity rights than those provided
under the Delaware General Corporation Law and the Company's Bylaws. The
indemnification agreements are not intended to deny or otherwise limit third
party or derivative suits against the Company or its directors or officers, but
to the extent a director or officer were entitled to indemnity or contribution
under the indemnification agreement, the financial burden of a third party suit
would be borne by the Company, and the Company would not benefit from derivative
recoveries against the director or officer. Such recoveries would accrue to the
benefit of the Company but would be offset by the Company's obligations to the
director or officer under the indemnification agreement.

         The above discussion of the Company's Bylaws, Restated Certificate of
Incorporation and indemnification agreements and of Section 145 of the Delaware
General Corporation Law is not intended to be exhaustive and is qualified in its
entirety by such Bylaws, Restated Certificate of Incorporation, indemnification
agreements and statute.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8.  EXHIBITS.

         The exhibits filed as part of this Registration Statement are as
follows:

                                      -2-
4

     EXHIBIT NO.  DESCRIPTION
     -----------  -----------

         4.1      Restated Certificate of Incorporation of Ecogen Inc. (Form
                  10-K for the fiscal year ended December 31, 1992)*

         4.2      Bylaws, as amended (Form S-1, Registration Statement, File No.
                  33-14119)*

         4.3      1999 Stock Option Plan (Form 10-Q for the fiscal quarter ended
                  April 30, 1999, filed on June 10, 1999)*

         5        Opinion of Paul, Hastings, Janofsky & Walker LLP

        23.1      Consent of KPMG LLP

        23.2      Consent of Paul, Hastings, Janofsky & Walker LLP (contained in
                  Exhibit 5)

        24.1      Power of Attorney (contained on the Signature Page of this
                  Registration Statement)

*        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 are hereby made a part of this Registration Statement.

ITEM 9.  UNDERTAKINGS.

         (a) We hereby undertake:

                  (1) That, for the purpose of determining any liability under
the Securities Act, each post-effective amendment to this Registration Statement
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial bona fide offering thereof.

                  (2) To remove from registration by means of a post-effective
amendment to this Registration Statement any of the securities being registered
which remain unsold at the termination of the offering.

         (b) We hereby undertake that, for purposes of determining any liability
under the Securities Act, each filing of our annual report pursuant to Section
13(a) or Section 15(d) of the Exchange Act that is incorporated by reference in
the Registration Statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

         (c) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to our directors, officers and controlling
persons pursuant to the foregoing provisions or otherwise, we have been advised
that in the opinion of the Commission such indemnification is against public
policy as expressed in the Securities Act and is, therefore, unenforceable. In
the event that a claim for indemnification against such liabilities (other than
the payment by us of expenses incurred or paid by our director, officer or
controlling person in the successful defense of any action, suit or proceeding)
is asserted by such director, officer or controlling person in connection with
the securities being registered, we will, unless in the opinion of our counsel
the matter has been settled by controlling precedent, submit to a court of
appropriate jurisdiction the question whether such indemnification by it is
against public policy as expressed in the Securities Act and will be governed by
the final adjudication of such issue.

                                      -3-
5

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, we certify that we
have reasonable grounds to believe that we meet all of the requirements for
filing on Form S-8 and have duly caused this Registration Statement to be signed
on our behalf by the undersigned, thereunto duly authorized, in Langhorne,
Commonwealth of Pennsylvania, on June 10, 1999.

                                   ECOGEN INC.

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

                               POWER OF ATTORNEY

         KNOW ALL MEN BY THESE PRESENTS, that each person whose signature
appears below constitutes and appoints James P. Reilly, Jr. and Mary E.
Paetzold, or either of them, his attorney-in-fact, each with the power of
substitution, for him in any and all capacities, to sign any amendments to this
Registration Statement, and to file the same, with exhibits thereto and other
documents in connection therewith, with the Securities and Exchange Commission,
hereby ratifying and confirming all that each of said attorneys-in-fact, or his
substitute or substitutes, may do or cause to be done by virtue thereof.

         Pursuant to the requirements of the Securities Act, this Registration
Statement has been signed by the following persons in the capacities and on the
dates indicated.
Signature                                   Title                               Date
---------                                   -----                               ----
/s/ James P. Reilly, Jr.            Chairman, Chief Executive Officer           June 10, 1999
------------------------            and Director (Principal Executive
James P. Reilly, Jr.                Officer)

/s/ Mary E. Paetzold
------------------------            Vice President and                          June 10, 1999
Mary E. Paetzold                    Chief Financial Officer
                                    (Principal Financial Officer
                                    and Principal Accounting Officer)

/s/ Esteban A. Ferrer
------------------------            Director                                    June 10, 1999
Esteban A. Ferrer

/s/ Philippe D. Katz
------------------------            Director                                    June 10, 1999
Philippe D. Katz

/s/ Lowell N. Lewis
------------------------            Director                                    June 10, 1999
Lowell N. Lewis

/s/ John R. Sutley
------------------------            Director                                    June 9, 1999
John R. Sutley
-4-
6

                                  EXHIBIT INDEX

     EXHIBIT NO.  DESCRIPTION
     -----------  -----------

          5       Opinion of Paul, Hastings, Janofsky & Walker LLP

         23.1     Consent of KPMG LLP

         23.2     Consent of Paul, Hastings, Janofsky & Walker LLP (contained in
                  Exhibit 5)

         24.1     Power of Attorney (contained on the Signature Page of this
                  Registration Statement)

                                      -5-