SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  Schedule 13D

                    Under the Securities Exchange Act of 1934
                                (Amendment No. 1)

                               Mycogen Corporation
                                (Name of Issuer)

                          Common Stock $ .001 Par Value
                         (Title of Class of Securities)

                                    628452104
                                 (CUSIP Number)

                          Dan Cornelison, 515/248-4823,
              400 Locust St., 700 Capital Sq., Des Moines, IA 50309
                  (Name, Address and Telephone number of person
                Authorized to Receive Notices and Communications)

                                December 2, 1996
             (Date of Event which Required Filing of this Statement)

If the filing person has previously  filed a statement on Schedule 13G to report
the  acquisition  which is the subject of this  Schedule 13D, and is filing this
schedule because of Rule 13D-1(b)(3) or (4), check the following box ____.

Check the following box if a fee is being paid with the statement  _____. (A fee
is not required only if the reporting  person:  (1) has a previous  statement on
file  reporting  beneficial  ownership of more than five percent of the class of
securities  described  in Item 1;  and (2) has  filed  no  amendment  subsequent
thereto reporting  beneficial  ownership of five percent or less of such class.)
(See Rule 13d-7.)

Note: Six copies of this statement, including all exhibits, should be filed with
the  Commission.  See Rule 13d -1(a) for other parities to whom copies are to be
sent.

*The  remainder  of this cover page shall be filed out for a reporting  person's
initial filing on this form with respect to the subject class of securities, and
for  any  subsequent   amendment   containing   information  which  would  alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the  Securities  Exchange  Act of
1934 ("Act") or otherwise subject the liabilities of that section of the Act but
shall be subject to all other provisions of the Act (however, see the Notes).




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1. Name of Reporting Person, S.S. or I.R.S. Identification No. of above Person:
   Pioneer Hi-Bred International, Inc. (shares held by a wholly owned subsidiary
   Pioneer Overseas Corporation, 42-1134927)  42-0470520
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2. Check the appropriate box if a member of a Group:  Not Applicable   a.____
                                                                       b.____
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3. SEC use only
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4. Source of Funds*:  00 (Company Funds)
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5. Check box if disclosure of legal proceedings is required pursuant to items
   2(d) or 2(e):  Not Applicable
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6. Citizenship or Place of Organization:  Iowa Corporation
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Number of shares      7.  Sole Voting Power:  2,000,000
                      ----------------------------------------------------------
beneficially owned    8.  Shared Voting Power:  Not Applicable
                      ----------------------------------------------------------
each reporting        9.  Sole Dispositive Power:  2,000,000
                      ----------------------------------------------------------
person with           10. Shares Dispositive Power:  Not Applicable
                      ----------------------------------------------------------

11. Aggregate Amount Beneficially Owned by each Reporting Person:  2,000,000
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12. Check box if the Aggregate amount in row (11) excluded certain shares*:____
    Not Applicable
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13. Percent of Class Represented by amount in row (11):  6.5
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14. Type Of reporting Person*:  CO




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                                  SCHEDULE 13D

Item 1.        Security and Issuer.
               This  Schedule  13D  relates  to  Common  Stock,  $.001 par value
("Common Stock"), of Mycogen Corporation,  a California Corporation ("Mycogen"),
the principal  executive offices of which are located at 5501 Oberlin Drive, San
Diego, California 92121.

Item 2.        Identity and Background.
               Pioneer Hi-Bred  International,  Inc. ("Pioneer"),  the reporting
person,  is an Iowa Corporation.  Pioneer develops,  produces and markets a full
line of seeds,  microbial products and services to farmers, grain processors and
other customers.  The address of its principal office is 400 Locust Street,  700
Capital Square, Des Moines, Iowa 50309. Pioneer owns the shares through a wholly
owned subsidiary,  Pioneer Overseas Corporation ("POC") an Iowa Corporation. POC
is a holding company for shares of companies owned by Pioneer.

               The information  required by Item 2 with respect to the executive
officers, and directors of Pioneer is as follows:

Charles S. Johnson,  President & Chief Executive Officer and Director of Pioneer
and Vice President and Director of POC

Jerry L. Chicoine,  Senior Vice President,  Chief Financial  Officer & Corporate
Secretary to the Board and Vice President and Director of POC

John D. James, Senior Vice President of Pioneer

Dr. Richard McConnell, Senior Vice President & Director of Research of Pioneer

Wayne L. Beck, Vice President-Supply Management of Pioneer

Carrol D. Bolen, Vice President-Legal and Governmental Affairs

Dr. Anthony J. Cavalieri, Vice President & Director of Trait and Technology
Development of Pioneer

Jack A. Cavanah, Vice President & Director of Corn Research of Pioneer

Dwight G. Dollison, Vice President & Treasurer of Pioneer and Treasurer of PO

Andre Faget, Vice President & Director of Operations/South Europe of Pioneer

Thomas M. Hanigan, Vice President & Director of Information Management/Business
Information Services of Pioneer

Brian G. Hart, Vice President & Corporate Controller of Pioneer and Assistant
Treasurer of POC

James R. Houser, Vice President & Director of Nutrition and Industry Markets of
Pioneer

Dr. Hector R.R. Laurence, Vice President & Director of Operations/S.A., C.A.,
Caribbean of Pioneer

Mary A. McBride, Vice President-Marketing of Pioneer

Dr.  James E.  Miller,  Vice  President & Director  of Oilseeds  and Field Crops
Research of Pioneer

Paul E. Schickler, Vice President & Director of Resource Planning of Pioneer

Harold F. Thorne, Vice President & Director of  Operations/Africa,  Middle East,
Asia, China of Pioneer

John T. Watson, Vice President & Director of Operations/CIS, Oceania, Turkey
of Pioneer and Vice President and Director of POC

Robert K. Wichmann, Vice President-North American Seed Sales of Pioneer

Doug Smith,  Corporate attorney for Pioneer and Assistant Secretary and Director
of POC

Michael A. Davis, Corporate attorney for Pioneer and Secretary of POC

The business  address of the above  persons is: 400 Locust  Street,  700 Capital
Square, Des Moines, IA 50309.






The following are Directors of Pioneer:

Dr. Ray A. Goldberg - Since July, 1970, Dr. Goldberg has been Moffett  Professor
of Agriculture  and Business,  Harvard  University  Graduate  School of Business
Administration.  His business address is: Harvard University, Graduate School of
Business Administration, Soldiers Field, Boston, MA 02163.

Dr. F. Warren McFarlan - Dr. McFarlan has been the Ross Graham Walker  Professor
of  Business  Administration,  Harvard  University  Graduate  School of Business
Administration  and  tenured  since  1973.  His  business  address  is:  Harvard
University,  Graduate School of Business  Administration,  Soldiers Field, Baker
185, Boston, MA 02163.

Dr.  Owen J.  Newlin - From 1978 to 1993,  Dr.  Newlin  served  in an  executive
position with the Company.  Dr. Newlin  retired as Senior Vice  President of the
Company in April,  1993. Dr. Newlin is a Director of Boatman's Bank,  Iowa, N.A.
and Central  Iowa Health  System (a  non-profit  hospital),  each of Des Moines,
Iowa. His business address is: 3315 48th Place, Des Moines, IA 50310.

Thomas N. Urban - Mr. Urban is  currently  Chairman of the Board of the Company.
Mr.  Urban is a Senior  Lecturer at Harvard  Graduate  School of  Business.  His
business   address  is:  Harvard   University,   Graduate   School  of  Business
Administration, Soldiers Field, Baker Library 186, Boston, MA 02163.

Dr.  Pedro M.  Cuatrecasas  - Since  1989,  Dr.  Cuatrecasas  has served as Vice
President of Warner-Lambert Company, Morris Plains, New Jersey (a pharmaceutical
company), and as President of its Pharmaceutical Research Division in Ann Arbor,
Michigan. His business address is: Parke-Davis Company, Pharmaceutical Division,
2800 Plymouth Road, Ann Arbor, MI 48106-1047.

Fred S.  Hubbell - Mr.  Hubbell  is  Chairman  and Chief  Executive  Officer  of
Equitable of Iowa  Companies,  Des Moines,  Iowa (a life insurance and annuities
company). His business address is: Equitable of Iowa Companies, 604 Locust, P.O.
Box 1635, Des Moines, IA 50306.

H. Scott  Wallace - Since  1992,  Mr.  Wallace  has been a criminal  justice and
government relations  consultant,  primarily as Special Counsel for the National
Legal Aid and Defender  Association  (a  nonprofit  educational  association  of
lawyers).  His business address is: 1625 "K" Street, Suite 800,  Washington,  DC
20006.

Herman  H.F.  Wijffels - Since  1986,  Mr.  Wijffels  has been  Chairman  of the
Executive Board of Rabobank  Nederland,  The Netherlands (a cooperative  banking
organization doing business internationally).  His business address is: Rabobank
Nederland, Croeselaan 18, Postbus 17100, 3500 HG Utrecht, The Netherlands.

Nancy Y. Bekavac - Since July,  1990,  Ms. Bekavac has been President of Scripps
College,  Claremont,  California. Her business address is: Scripps College, 1030
Columbia Avenue, Claremont, CA 91711.

C. Robert  Brenton - Since 1990,  Mr.  Brenton has been Chairman of the Board of
Brenton Banks,  Inc.,  and is currently a Director of Brenton  Banks,  Inc., Des
Moines,  Iowa. His business address is: Brenton Banks, Inc. Box 961, Des Moines,
IA 50304.

Luiz Kaufmann - Since  November,  1993, Mr.  Kaufmann has been the President and
CEO of Aracruz  Celulose S.A.,  Rio de Janeiro,  Brazil (a pulp  producer).  His
business address is: Aracruz  Celulose S.A., Rua Lauro Muller,  116, 40th Floor,
Rio de Janeiro, Brazil.

Dr.  Virginia  Walbot - Since 1989,  Dr. Walbot has been a Professor at Stanford
University's  Department  of  Biological  Sciences,  Stanford,  California.  Her
business  address is: Stanford  University,  Department of Biological  Sciences,
Stanford, CA 94305.

Fred W. Weitz - Mr. Weitz is President of Essex Meadows,  Inc., Des Moines, Iowa
(an operator of proprietary  retirement communities and owner of commercial real
estate).  From  1964  to  1995,  Mr.  Weitz  was  the  President  of  The  Weitz
Corporation,   Des  Moines,  Iowa  (a  building  construction  and  real  estate
development company).  His business address is: Essex Meadows,  Inc., 800 Second
Avenue, Des Moines, IA 50309.

        None of Pioneer,  POC,  nor any of the  persons  named  above,  has been
convicted of any  criminal  offense  (excluding  traffic  violations  or similar
misdemeanors) during the past five years.

        None of Pioneer,  POC, nor any of the persons  named  above,  has been a
party  during  the  past  five  years to a civil  proceeding  of a  judicial  or
administrative body of competent jurisdiction and as a result of such proceeding
been or is subject to a judgment, decree or final order finding violations of or
enjoining future violations of or prohibiting or mandating activities subject to
federal or state securities laws.

        All persons named above are U.S. citizens except Herman Wijffels of The
Netherlands; Luiz Kaufmann of Brazil; Andre Faget of France; and Hector Laurence
of Argentina.

Item 3. Source and Amount of Funds or Other Consideration.
               The stock was purchased for $30 million with Pioneer funds.

Item 4.        Purpose Of Transaction
               The purpose of the  transaction  is for investment and is related
to a research  collaboration.  The  research  collaboration  will focus on using
Bacillus thuringiensis (BT) genes in crops to protect them against pest damage.



               The shares of common stock were purchased by Pioneer from Mycogen
on  December  13,  1995  pursuant  to a  stock  purchase  agreement  and are not
registered and are thus "restricted securities". In Connection with the purchase
of the shares of Common Stock  Pioneer and Mycogen  entered into a  Registration
Rights  Agreement,  dated December 13, 1995. The  Registration  Rights Agreement
gives Pioneer the right at any time after the first  anniversary of the purchase
to demand two registrations. Pioneer may also participate in any registration by
Mycogen for its own account or the account of others.

Item 5. Interest in Securities of the Issuer.
               (a) As of December, 1995, Pioneer through POC beneficially owns 3
million shares of Common Stock of Mycogen.  Based upon the shares outstanding as
shown in the Mycogen 10 K for the fiscal year ended August 31, 1995, Pioneer has
15.4% of the Common  Stock.  None of the other  persons  named in Item 2 of this
Schedule 13D beneficially  owns any shares of Common Stock (or warrants or other
rights to acquire such shares).
               (b)  Pioneer  has the sole  power  to vote and the sole  power to
dispose of the 3,000,000 shares of Common Stock beneficially owned by it.
               (c) Other  than the  purchase  reported  on  herein,  none of the
persons named in Item 2 of this Schedule 13D,  including  Pioneer,  has effected
during the past 60 days any transaction in the Common Stock of Mycogen.
               (d)  Not applicable
               (e)  Not applicable

Item 6. Contracts, Arrangements, Understandings or Relationships with        
        Respect to Securities of the Issuer.
               In  connection  with the  purchase of the shares of Common  Stock
Pioneer and Mycogen entered into a Registration Rights Agreement, dated December
13, 1995. The Registration  Rights Agreement gives Pioneer the right at any time
after the first anniversary of the purchase to demand two registrations. Pioneer
may also  participate in any  registration by Mycogen for its own account or the
account of others.

Item 7. Material to be filed as Exhibits.
               The Registration Rights Agreement is filed as Exhibit A, hereto.



<PAGE>


        After  reasonable  inquiry and to the best of its  knowledge and belief,
the  undersigned  certifies that the  information set forth in this statement is
true, complete and correct.
                       PIONEER HI-BRED INTERNATIONAL, INC.


                     By: /s/ Jerry L. Chicoine    Date:12/2/96
                     -------------------------    ------------
                             Jerry L. Chicoine
                             Senior Vice President




















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                                  Exhibit Index

                                                                 Page #
(a) Registration Rights agreement dated December 13, 1995        ______