UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  Schedule 13D

                    Under the Securities Exchange Act of 1934
                    -----------------------------------------
                                (Amendment No. 3)
                                -----------------

                               Mycogen Corporation
                               -------------------
                                (Name of Issuer)

           Common Stock $ .001 Par Value (Including the Associated Rights)
                         (Title of Class of Securities)

                                   628452 10 4
                                   -----------
                                 (CUSIP Number)

             William DeMeulenaere, 400 Locust St., 800 Capital Sq.,
             ------------------------------------------------------
                   Des Moines, IA 50309 (515/248-4820) (Name,
                     Address and Telephone number of person
                Authorized to Receive Notices and Communications)

                                 March 13, 1998
                                 --------------
           (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.)


This  Schedule  13D  contains  10  sequentially  numbered  pages.  There  are no
exhibits.


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CUSIP No.  628452 10 4                      13D
           -----------
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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*:  Not Applicable
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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:     None
                      ----------------------------------------------------------
beneficially owned    8.  Shared Voting Power:     Not Applicable
                      ----------------------------------------------------------
each reporting        9.  Sole Dispositive Power:     None
                      ----------------------------------------------------------
person with           10. Shares Dispositive Power:     Not Applicable
                      ----------------------------------------------------------

11. Aggregate Amount Beneficially Owned by each Reporting Person:     None
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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): 0.0%
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14.  Type Of reporting Person*:     CO





                                       2
<PAGE>



                                  SCHEDULE 13D

     This Amendment No. 3 is being filed to report the decrease in the reporting
person's ownership from -2,000,000- shares to -0- shares. It amends the original
13D filed by the reporting person in December, 1995, as amended by Amendment No.
1 filed on December 3, 1996 and as amended by Amendment  No. 2 filed on December
5, 1996.

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  worldwide.  The address of its principal  office is 400 Locust
Street,  800 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, Chairman  of the Board, President & Chief Executive Officer 
        and a Director of Pioneer and  President & Director of POC

Jerry L. Chicoine, Executive Vice President, Chief Operating Officer & Corporate
        Secretary and a Director of
        Pioneer and Vice President & Director of POC

John D. James, Senior Vice President of Pioneer
        Dr. Richard McConnell, Senior Vice Presiden  & Director of Research and 
        Product Development of Pioneer

Dr. Richard Mcconnell, Senior Vice President & Director of Research and Product
        Development of Pioneer

Mary A. McBride, Vice President - Marketing of Pioneer

                                      3
<PAGE>

Peg Armstrong-Gustafson, Vice President, Director--Worldwide Product Marketing

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

Carrol D. Bolen, Vice President of Pioneer

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

Jack A. Cavanah,  Vice  President  &  Director  of Product Characterization and 
        Commercialization of Pioneer

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

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

Brian G. Hart, Vice President & Chief Financial Officer of Pioneer  and Director
        of POC

James R. Houser, Vice President & Director of European Operations of Pioneer

Herbert H. Jervis, Vice President and Chief Patent Counsel of Pioneer

Dr. Hector R.R. Laurence, Vice President & Operations Director of Latin America
        of Pioneer

Dr. James E. Miller, Vice President & Director of Research--Product  Development
        of Pioneer

Paul E. Schickler, Vice President & Director of Administration of Pioneer

Leon R. Shearer,  Vice President &  General Counsel of Pioneer and Secretary of 
        POC 

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

John T. Watson, Vice President & Assistant Operations Director of Africa, Asia, 
        Middle East and China of Pioneer and Vice President & Director of POC

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

Duane A. Suess, Corporate Controller of Pioneer and Director & Controller of POC

                                      4
<PAGE>

The business address of the above persons is:  
     400 Locust Street, 800 Capital Square, Des Moines, IA  50309.
                                  
              The  following  persons are  Directors of Pioneer, in addition to 
Mr. Johnson and Mr. Chicoine listed above:

Dr. F. Warren McFarlan - Dr. McFarlan has been the Albert E. Gordon Professor of
Business   Administration,   Harvard  University  Graduate  School  of  Business
Administration  and tenured since 1973. Dr.  McFarlan is a Director of Providian
Financial  Corporation,  San  Francisco,  California (a credit card company) and
Computer  Sciences  Corporation,  Los  Angeles,  California  (a computer  system
integration  company).  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 the  President  of the Board of Regents
State  of Iowa,  Des  Moines,  Iowa  (the  governing  body of the  three  public
universities and the two special schools in the state of Iowa) and a Director of
NationsBank,  Iowa,  N.A.,  Des Moines,  Iowa (a national  bank).  His  business
address is: 400 Locust Street, Suite 800, Des Moines, IA 50309.

Thomas N. Urban - Mr.  Urban served as Chairman of the Board of the Company from
1984 to December 1996 and served as President  between 1984 and March 1995.  Mr.
Urban is also a Director of ING America Insurance  Holdings,  Inc.,  Atlanta, GA
(an insurance  holding  company);  Sigma Aldrich  Corporation,  St. Louis, MO (a
research  chemicals   company);   and  The  Case  Corporation,   Racine,  WI  (a
construction and agricultural  equipment company).  His business address is: 400
Locust Street, Suite 800, Des Moines, IA 50309.

                                     5
<PAGE>

Dr.  Pedro  M.   Cuatrecasas  -  Dr.   Cuatrecasas  has  been  a  consultant  in
pharmaceutical  and health sciences,  operating out of Rancho Santa Fe, CA since
January 1997. He also is a consultant for Warner-Lambert Company, Morris Plains,
New Jersey (a pharmaceutical company). His business address is: 6039 Lago Linda,
Rancho Sante Fe, CA 92067.
                             
Fred S.  Hubbell - Mr.  Hubbell is Chairman and Chief  Executive  Officer of U S
Life and  Annuity  Companies,  ING Group North  America of  Atlanta,  GA (a life
insurance  and annuities  company).  He also serves as a Director of ING America
Insurance  Holdings,  Inc.,  Atlanta,  GA (an insurance  holding  company).  His
business  address is: U S Life and Annuity  Companies,  ING Group North America,
909 Locust, P.O. Box 1635, Des Moines, IA 50306.

H. Scott Wallace - Mr. Wallace is the Director of the Division of Defender Legal
Services  for the  National  Legal Aid and  Defender  Association  (a  nonprofit
educational association of lawyers),  Washington,  D.C. His business address is:
800 Locust Street, Suite 400, Des Moines, IA 50309.

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. Ms. Bekavac is also a Director of Electro Rent
Corp.,  Van Nuys, CA (a computer and electronic test and  measurement  equipment
rental company). 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., 400 Locust Street,
Suite 300, Box 961, Des Moines, IA 50304.

                                  6
<PAGE>

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, 22nd 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 - Since 1995 Mr. Weitz has been 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.

Charles O.  Holliday,  Jr. - Mr.  Holliday is the  President and a member of the
Board of Directors  of E.I.  DuPont  deNemours  and Company  ("DuPont").  He has
served in various  capacities with DuPont and its affiliates since 1990. He also
is a director of DuPont  Photomasks,  Inc., Round Rock, TX (a global  photomask,
photoblanks and pellicles  manufacturer) and a director of Analog Devices, Inc.,
Norwood MA (an integrated circuit  manufacturer).  His business address is: 1007
Market Street, D-9000, Wilmington, DE 19898.

William F. Kirk - Mr. Kirk is Senior Vice President of DuPont and served as Vice
President  and  General  Manager of DuPont  Agricultural  Products  from 1990 to
November 1997. His business  address is: Barley Mill Plaza No. 38, Route 141 and
Lancaster Pike, Wilmington, DE 19805.

                                      7
<PAGE>

                                  
        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; and Hector Laurence of Argentina.

Item 3. Source and Amount of Funds or Other Consideration.
------  --------------------------------------------------
               Not applicable

Item 4.        Purpose Of Transaction
-------        ----------------------
               On  March  13,  1998  POC  agreed  to  sell  to Dow  AgroSciences
2,000,000 shares of Common Stock of Mycogen  Corporation  ("Mycogen") at a price
of $20.059 per share,  subject to approval by the Executive committee of The Dow
Chemical Company.  The Executive Committee approved the transaction on March 16,
1998 and the purchase was  consummated  on March 17, 1998.  The purpose that POC
has in selling  its shares to Dow  AgroSciences  is to sell its shares at prices
and on terms that Pioneer considers favorable.
                                 
Item 5. Interest in Securities of the Issuer.
---------------------------------------------
               (a) As of March 17, 1998 and upon the consummation of the sale of
shares to Dow  AgroSciences,  Pioneer  does not  beneficially  own any shares of


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<PAGE>

Common  Stock of  Mycogen.  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) Not applicable.
               (c) Other than the sale  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
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        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 gave 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. The Registration Rights Agreement is now void.

Item 7. Material to be filed as Exhibits.
-----------------------------------------
               The Registration  Rights Agreement was filed as Exhibit A, to the
original Schedule 13D filing in December 1995.





                                      9
<PAGE>




        After  reasonable  inquiry and to the best of my knowledge and belief, I
certify that the information  set forth in this statement is true,  complete and
correct.

                       PIONEER HI-BRED INTERNATIONAL, INC.
                       By: /s/Brian G. Hart Date: 3/17/98
                              Brian G. Hart
                              Vice President and Chief Financial Officer