LETTER OF TRANSMITTAL
                        TO TENDER SHARES OF COMMON STOCK
           (INCLUDING THE ASSOCIATED PREFERRED STOCK PURCHASE RIGHTS)
                                       OF
                              MYCOGEN CORPORATION
           PURSUANT TO THE OFFER TO PURCHASE DATED SEPTEMBER 4, 1998
                                       OF
                         AGROSCIENCES ACQUISITION INC.
                         A MAJORITY-OWNED SUBSIDIARY OF
                              DOW AGROSCIENCES LLC
 
                   AND A WHOLLY OWNED INDIRECT SUBSIDIARY OF
                            THE DOW CHEMICAL COMPANY
      -------------------------------------------------------------------
      THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
                                     TIME,
            ON FRIDAY, OCTOBER 2, 1998, UNLESS THE OFFER IS EXTENDED.
- --------------------------------------------------------------------------------
 
To: BankBoston, N.A., Depositary
 

                                                                    
           BY MAIL:                             BY HAND:                    BY OVERNIGHT, CERTIFIED OR EXPRESS MAIL
       BankBoston, N.A.             Securities Transfer & Reporting                        DELIVERY:
Attn: Corporate Reorganization               Services, Inc.                             BankBoston, N.A.
        P.O. Box 8029                   c/o Boston EquiServe LP                  Attn: Corporate Reorganization
    Boston, Massachusetts                   1 Exchange Plaza                           150 Royall Street
          02266-8029                     55 Broadway, 3rd Floor                   Canton, Massachusetts 02021
                                        New York, New York 10006

 

                           
       BY FACSIMILE:              CONFIRM FACSIMILE BY
    (781) 575-2233/2232                TELEPHONE:
 (For Eligible Institutions          (800) 730-4001
           Only)                 (For Confirmation Only)

 
  DELIVERY OF THIS LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH
   ABOVE OR TRANSMISSION OF INSTRUCTIONS VIA A FACSIMILE TRANSMISSION TO A
       NUMBER OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID
      DELIVERY. THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL
       SHOULD BE READ CAREFULLY BEFORE THIS LETTER OF TRANSMITTAL IS
                                   COMPLETED.
 
    This Letter of Transmittal is to be used by stockholders of Mycogen
Corporation if certificates for Shares (as defined below) are to be forwarded
herewith or, unless an Agent's Message (as defined in the Offer to Purchase) is
utilized, if delivery of Shares is to be made by book-entry transfer to the
Depositary's account at The Depository Trust Company (the "Book-Entry Transfer
Facility" or "DTC") pursuant to the book-entry transfer procedures set forth
under "The Tender Offer--3. Procedure for Tendering Shares" in the Offer to
Purchase dated September 4, 1998. Stockholders who tender Shares by book-entry
transfer are referred to herein as "Book-Entry Stockholders." Delivery of
documents to the Book-Entry Transfer Facility does not constitute delivery to
the Depositary.
 
    Stockholders who cannot deliver their Shares and all other documents
required hereby to the Depositary on or prior to the Expiration Date (as defined
in the Offer to Purchase) or who cannot complete the procedures for book-entry
transfer on a timely basis, must tender their Shares pursuant to the guaranteed
delivery procedure set forth under "The Tender Offer--3. Procedure for Tendering
Shares" in the Offer to Purchase. See Instruction 2.


- ---------------
                                                DESCRIPTION OF SHARES TENDERED
                                                  (SEE INSTRUCTIONS 2 AND 4)
 ------------
                                                                NAMES(S) AND ADDRESS(ES) OF REGISTERED HOLDER(S)
                 SHARES TENDERED                        (PLEASE FILL IN, IF BLANK, EXACTLY AS NAME(S) APPEAR(S) ON SHARE
     (ATTACH ADDITIONAL LIST, IF NECESSARY)                                       CERTIFICATES)
                                          
- ---------------
 

                 TOTAL NUMBER OF
                     SHARES          NUMBER OF
  CERTIFICATE    REPRESENTED BY       SHARES
  NUMBER(S)*     CERTIFICATE(S)*    TENDERED**
                                          
- ---------------
 
- ---------------
 
- ---------------
 
- ---------------
 
- ---------------
 
- ---------------
 
- ---------------
 
- ---------------
 TOTAL SHARES
- ---------------
*   Need not be completed by stockholders tendering by book-entry transfer.
**  Unless otherwise indicated, it will be assumed that all Shares represented by any certificates delivered to the Depositary
    are being tendered. See Instruction 4.
 -------------

 

                    NOTE: SIGNATURES MUST BE PROVIDED BELOW
                PLEASE READ ACCOMPANYING INSTRUCTIONS CAREFULLY
 


   -------------------------------------------------------------------------------------------
 
/ /  CHECK HERE IF TENDERED SHARES ARE BEING DELIVERED BY BOOK-ENTRY TRANSFER TO THE DEPOSITARY'S
     ACCOUNT AT THE BOOK-ENTRY TRANSFER FACILITY AND COMPLETE THE FOLLOWING:
        
 
Name of Tendering Institution:
DTC Account No.:
DTC Transaction Code No.:
/ /  CHECK HERE IF TENDERED SHARES ARE BEING DELIVERED PURSUANT TO A NOTICE OF GUARANTEED DELIVERY
     PREVIOUSLY SENT TO THE DEPOSITARY AND COMPLETE THE FOLLOWING:
 
Name(s) of Registered Stockholder(s):
Window Ticket Number (if any):
Date of Execution of Notice of Guaranteed Delivery:
Name of Institution which Guaranteed Delivery:
If delivery is by book entry transfer:
           Name of Tendering Institution:
           DTC Account No.:
           DTC Transaction Code No.:
- -------------------------------------------------------------------------------------------

 
Ladies and Gentlemen:
 
    The undersigned hereby tenders to AgroSciences Acquisition Inc., a Delaware
corporation ("Purchaser") and a majority-owned subsidiary of Dow AgroSciences
LLC, the above-described shares of common stock, par value $0.001 per share
(including the associated preferred stock purchase rights) (the "Shares"), of
Mycogen Corporation, a California corporation (the "Company"), pursuant to
Purchaser's offer to purchase all of the outstanding Shares at a price of $28.00
per Share, net to the seller in cash, without interest thereon, upon the terms
and subject to the conditions set forth in the Offer to Purchase dated September
4, 1998 (the "Offer to Purchase"), receipt of which is hereby acknowledged, and
in this Letter of Transmittal (which together, as may be amended or supplemented
from time to time, constitute the "Offer").
 
    Subject to, and effective upon, acceptance for payment of the Shares
tendered herewith in accordance with the terms of the Offer, including, if the
Offer is extended or amended, the terms and conditions of any such extension or
amendment, the undersigned hereby sells, assigns and transfers to or upon the
order of Purchaser all right, title and interest in and to all the Shares that
are being tendered hereby or orders the registration of such Shares delivered by
book-entry transfer (and any and all other Shares or other securities issued or
issuable in respect thereof on or after September 4, 1998, and any or all
dividends thereon or distributions with respect thereto (collectively,
"Distributions")) and irrevocably appoints the Depositary the true and lawful
agent and attorney-in-fact of the undersigned with respect to such Shares (and
all Distributions), with full power of substitution (such power of attorney
being deemed to be an irrevocable power coupled with an interest), to (a)
deliver certificates for such Shares (and all Distributions), or transfer
ownership of such Shares (and all Distributions) on the account books maintained
by the Book-Entry Transfer Facility, together, in any such case, with all
accompanying evidences of transfer and
 
                                       2

authenticity to, or upon the order of, Purchaser upon receipt by the Depositary,
as the undersigned's agent, of the purchase price, (b) present such Shares (and
all Distributions) for transfer on the books of the Company and (c) receive all
benefits and otherwise exercise all rights of beneficial ownership of such
Shares (and all Distributions), all in accordance with the terms of the Offer.
 
    The undersigned hereby irrevocably appoints Charles J. Hahn, Jane M. Gootee
and Brian G. Taylorson and each of them, the attorneys-in-fact and proxies of
the undersigned, each with full power of substitution, to exercise all voting
and other rights of the undersigned in such manner as each such attorney and
proxy or his substitute shall in his sole discretion deem proper, with respect
to all of the Shares (and all Distributions) tendered hereby which have been
accepted for payment by Purchaser prior to the time of any vote or other action
at any meeting of stockholders of the Company (whether annual or special and
whether or not an adjourned meeting), by written consent or otherwise. This
power of attorney and proxy is coupled with an interest and is irrevocable and
is granted in consideration of, and is effective upon, the acceptance for
payment of such Shares by Purchaser in accordance with the terms of the Offer.
Such acceptance for payment shall revoke, without any further action, any other
power of attorney or proxy granted by the undersigned at any time with respect
to such Shares, and no subsequent power of attorney or proxies will be given or
will be executed by the undersigned (and if given or executed, will not be
deemed to be effective). The undersigned understands that Purchaser reserves the
right to require that, in order for such Shares to be deemed validly tendered,
immediately upon Purchaser's acceptance for payment of such Shares, Purchaser is
able to exercise full voting rights with respect to such Shares and other
securities, including voting at any meeting of stockholders.
 
    The undersigned hereby represents and warrants that the undersigned has full
power and authority to tender, sell, assign and transfer the Shares and all
Distributions tendered hereby and that when the same are accepted for payment by
Purchaser, Purchaser will acquire good and marketable title and unencumbered
ownership thereto, free and clear of all liens, restrictions, charges, security
interests, and encumbrances and not subject to any adverse claims. The
undersigned will, upon request, execute and deliver any additional documents
deemed by the Depositary or Purchaser to be necessary or desirable to complete
the sale, assignment and transfer of the Shares and all Distributions tendered
hereby. In addition, the undersigned will promptly remit and transfer to the
Depositary for the account of Purchaser any and all Distributions in respect of
the Shares tendered hereby, accompanied by appropriate documentation of transfer
and, pending such remittance or appropriate assurance thereof, Purchaser shall
be entitled to all rights and privileges as owner of any such Distributions, and
may withhold the entire purchase price or deduct from the purchase price of
Shares tendered hereby, the amount or value thereof, as determined by Purchaser
in its sole discretion.
 
    All authority herein conferred or agreed to be conferred shall survive the
death or incapacity of the undersigned, and any obligation of the undersigned
hereunder shall be binding upon the heirs, personal representatives, successors
and assigns of the undersigned. Except as stated in the Offer or otherwise
required by applicable law, this tender is irrevocable.
 
    The undersigned understands that tenders of Shares pursuant to any one of
the procedures described under "The Tender Offer--3. Procedure for Tendering
Shares" in the Offer to Purchase and in the instructions hereto will constitute
a binding agreement between the undersigned and Purchaser upon the terms and
subject to the conditions of the Offer.
 
    The undersigned recognizes that, under certain circumstances set forth in
the Offer to Purchase, Purchaser may terminate or amend the Offer or may not be
required to accept for payment any of the Shares tendered herewith.
 
    Unless otherwise indicated under "Special Payment Instructions," please
issue the check for the purchase price or return any Shares not tendered or
accepted for payment in the name(s) of the undersigned. Similarly, unless
otherwise indicated under "Special Delivery Instructions," please mail the check
for the purchase price or return any Share certificates not tendered or accepted
for payment (and accompanying documents, as appropriate) to the undersigned at
the address shown below the undersigned's signature(s). In the event that both
"Special Payment Instructions" and "Special Delivery Instructions" are
completed, please issue the check for the purchase price or return any Shares
not tendered or accepted for payment in the name(s) of, and deliver said check
or return certificates to, the
 
                                       3

person or persons so indicated. Stockholders tendering Shares by book-entry
transfer may request that any Shares not accepted for payment be returned by
crediting such account maintained at the Book-Entry Transfer Facility as such
stockholder may designate by making an appropriate entry under "Special Payment
Instructions." The undersigned recognizes that Purchaser has no obligation
pursuant to the "Special Payment Instructions" to transfer any Shares from the
name of the registered holder thereof if Purchaser does not accept for payment
any of such Shares.
 
                                  INSTRUCTIONS
             FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER
 
    1. GUARANTEE OF SIGNATURES.  Except as otherwise provided below, all
signatures on this Letter of Transmittal must be guaranteed by a member firm of
a registered national securities exchange, a member of the National Association
of Securities Dealers, Inc., a financial institution (including most banks,
trust companies, savings and loan associations and brokerage houses) which is a
participant in the Securities Transfer Agents Medallion Program, the New York
Stock Exchange Medallion Signature Program or the Stock Exchange Medallion
Program (an "Eligible Institution"). Signatures on this Letter of Transmittal
need not be guaranteed (a) if this Letter of Transmittal is signed by the
registered holder(s) of the Shares (which term, for purposes of this document,
shall include any participant in the Book-Entry Transfer Facility whose name
appears on a security position listing as the owner of Shares) tendered herewith
and such holder(s) have not completed the instruction entitled "Special Delivery
Instruments" or "Special Payment Instructions" on this Letter of Transmittal or
(b) if such Shares are tendered for the account of an Eligible Institution. See
Instruction 5.
 
    2. DELIVERY OF LETTER OF TRANSMITTAL AND SHARES.  This Letter of Transmittal
is to be used either if certificates are to be forwarded herewith or if Shares
are to be delivered by book-entry transfer pursuant to the procedures set forth
in "The Tender Offer--3. Procedure for Tendering Shares" of the Offer to
Purchase. Certificates for all physically delivered Shares, or a confirmation of
a book-entry transfer into the Depositary's account at the Book-Entry Transfer
Facility of all Shares delivered electronically, as well as a properly completed
and duly executed Letter of Transmittal (or facsimile thereof) (or, in the case
of a book-entry delivery, an Agent's Message) and any other documents required
by this Letter of Transmittal, must be received by the Depositary at one of its
addresses set forth on the front page of this Letter of Transmittal on or prior
to the Expiration Date. Stockholders who cannot deliver their Shares and all
other required documents to the Depositary on or prior to the Expiration Date
must tender their Shares pursuant to the guaranteed delivery procedure set forth
in "The Tender Offer--3. Procedure for Tendering Shares" of the Offer to
Purchase. Pursuant to such procedure: (a) such tender must be made by or through
an Eligible Institution, (b) a properly completed and duly executed Notice of
Guaranteed Delivery substantially in the form provided by AgroSciences
Acquisition Inc. ("Purchaser") must be received by the Depositary on or prior to
the Expiration Date and (c) the certificates for all physically delivered
Shares, or a confirmation of a book-entry transfer into the Depositary's account
at the Book-Entry Transfer Facility of all Shares delivered electronically, as
well as a properly completed and duly executed Letter of Transmittal (or
facsimile thereof) (or, in the case of a book-entry delivery, an Agent's
Message) and any other documents required by this Letter of Transmittal, must be
received by the Depositary within three NASDAQ National Market System trading
days after the date of execution of such Notice of Guaranteed Delivery, all as
provided in "The Tender Offer--3. Procedure for Tendering Shares." If Shares are
forwarded separately to the Depositary, each must be accompanied by a duly
executed Letter of Transmittal (or facsimile thereof).
 
    THE METHOD OF DELIVERING SHARES, THE LETTER OF TRANSMITTAL AND ALL OTHER
REQUIRED DOCUMENTS INCLUDING DELIVERY THROUGH THE BOOK-ENTRY TRANSFER FACILITY,
IS AT THE OPTION AND SOLE RISK OF THE TENDERING STOCKHOLDER AND THE DELIVERY
WILL BE DEEMED MADE ONLY WHEN ACTUALLY RECEIVED BY THE DEPOSITARY. IF DELIVERY
IS BY MAIL, REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED, IS
RECOMMENDED. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY
DELIVERY.
 
                                       4

    No alternative, conditional or contingent tenders will be accepted. By
executing this Letter of Transmittal (or facsimile thereof), the tendering
stockholder waives any right to receive any notice of the acceptance for payment
of the Shares.
 
    3. INADEQUATE SPACE.  If the space provided herein is inadequate, the
certificate numbers or the number of Shares should be listed on a separate
schedule attached hereto and separately signed on each page thereof in the same
manner as this Letter of Transmittal is signed.
 
    4. PARTIAL TENDERS (NOT APPLICABLE TO STOCKHOLDERS WHO TENDER BY BOOK-ENTRY
TRANSFER).  If fewer than all the Shares represented by any certificate
delivered to the Depositary are to be tendered, fill in the number of Shares
which are to be tendered in the box entitled "Number of Shares Tendered." In
such case, a new certificate for the remainder of the Shares represented by the
old certificate will be sent to the person(s) signing this Letter of
Transmittal, unless otherwise provided in the appropriate box on this Letter of
Transmittal, as promptly as practicable following the expiration or termination
of the Offer. All Shares represented by certificates delivered to the Depositary
will be deemed to have been tendered unless otherwise indicated.
 
    5. SIGNATURES ON LETTER OF TRANSMITTAL; STOCK POWERS AND ENDORSEMENTS.  If
this Letter of Transmittal is signed by the registered holder(s) of the Shares
tendered hereby, the signature(s) must correspond with the name(s) as written on
the face of the certificates without alteration, enlargement or any change
whatsoever.
 
    If any of the Shares tendered hereby are held of record by two or more
persons, all such persons must sign this Letter of Transmittal.
 
    If any of the Shares tendered hereby are registered in different names on
different certificates, it will be necessary to complete, sign and submit as
many separate Letters of Transmittal as there are different registrations of
certificates.
 
    If this Letter of Transmittal is signed by the registered holder(s) of the
Shares tendered hereby, no endorsements of certificates or separate stock powers
are required unless payment of the purchase price is to be made, or Shares not
tendered or not purchased are to be returned, in the name of any person other
than the registered holder(s). Signatures on any such certificates or stock
powers must be guaranteed by an Eligible Institution.
 
    If this Letter of Transmittal is signed by a person other than the
registered holder(s) of the Shares tendered hereby, certificates must be
endorsed or accompanied by appropriate stock powers, in either case, signed
exactly as the name(s) of the registered holder(s) appear(s) on the certificates
for such Shares. Signature(s) on any such certificates or stock powers must be
guaranteed by an Eligible Institution.
 
    If this Letter of Transmittal or any certificate or stock power is signed by
a trustee, executor, administrator, guardian, attorney-in-fact, officer of a
corporation or other person acting in a fiduciary or representative capacity,
such person should so indicate when signing, and proper evidence satisfactory to
Purchaser of the authority of such person so to act must be submitted.
 
    6. STOCK TRANSFER TAXES.  Except as provided in this Instruction 6,
Purchaser will pay any stock transfer taxes with respect to the sale and
transfer of any Shares to it or its order pursuant to the Offer. If, however,
payment of the purchase price is to be made to, or Shares not tendered or not
purchased are to be returned in the name of, any person other than the
registered holder(s), or if a transfer tax is imposed for any reason other than
the sale or transfer of Shares to Purchaser pursuant to the Offer, then the
amount of any stock transfer taxes (whether imposed on the registered holder(s),
such other person or otherwise) will be deducted from the purchase price unless
satisfactory evidence of the payment of such taxes, or exemption therefrom, is
submitted herewith.
 
    EXCEPT AS PROVIDED IN THIS INSTRUCTION 6, IT WILL NOT BE NECESSARY FOR
TRANSFER TAX STAMPS TO BE AFFIXED TO THE CERTIFICATES LISTED IN THIS LETTER OF
TRANSMITTAL.
 
    7. SPECIAL PAYMENT AND DELIVERY INSTRUCTIONS.  If the check for the purchase
price of any Shares purchased is to be issued, or any Shares not tendered or not
purchased are to be returned, in the name of a person other than the person(s)
signing this Letter of Transmittal or if the check or any certificates for
 
                                       5

Shares not tendered or not purchased are to be mailed to someone other than the
person(s) signing this Letter of Transmittal or to the person(s) signing this
Letter of Transmittal at an address other than that shown above, the appropriate
boxes on this Letter of Transmittal should be completed. Stockholders tendering
Shares by book-entry transfer may request that Shares not purchased be credited
to such account at the Book-Entry Transfer Facility as such stockholder may
designate under "Special Payment Instructions." If no such instructions are
given, any such Shares not purchased will be returned by crediting the account
at the Book-Entry Transfer Facility designated above.
 
    8. SUBSTITUTE FORM W-9.  Under the federal income tax laws, the Depositary
will be required to backup withhold 31% of the amount of any payments made to
certain stockholders pursuant to the Offer. In order to avoid such backup
withholding, each tendering stockholder, and, if applicable, each other payee,
must provide the Depositary with such stockholder's or payee's correct taxpayer
identification number and certify that such stockholder or payee is not subject
to such backup withholding by completing the Substitute Form W-9 set forth
below. In general, if a stockholder or payee is an individual, the taxpayer
identification number is the Social Security number of such individual. If the
Depositary is not provided with the correct taxpayer identification number, the
stockholder or payee may be subject to a $50 penalty imposed by the Internal
Revenue Service ("IRS"). Certain stockholders or payees (including, among
others, all corporations and certain foreign individuals) are not subject to
these backup withholding and reporting requirements. In order to satisfy the
Depositary that a foreign individual qualifies as an exempt recipient, such
stockholder or payee must submit a statement, signed under penalties of perjury,
attesting to that individual's exempt status. Such statements can be obtained
from the Depositary. For further information concerning backup withholding and
instructions for completing the Substitute Form W-9 (including how to obtain a
taxpayer identification number if you do not have one and how to complete the
Substitute Form W-9 if Shares are held in more than one name), consult the
enclosed GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON
SUBSTITUTE FORM W-9.
 
    Failure to complete the Substitute Form W-9 will not, by itself, cause
Shares to be deemed invalidly tendered, but may require the Depositary to
withhold 31% of the amount of any payments made pursuant to the Offer. Backup
withholding is not an additional federal income tax. Rather, the federal income
tax liability of a person subject to backup withholding will be reduced by the
amount of tax withheld. If withholding results in an overpayment of taxes, a
refund may be obtained provided that the required information is furnished to
the IRS.
 
    If a tendering stockholder has not been issued a TIN and has applied for a
TIN or intends to apply for a TIN in the near future, such holder must complete
the Certificate Awaiting Taxpayer Identification Number below in order to avoid
backup withholding. Notwithstanding that the Certificate of Awaiting Taxpayer
Identification Number is completed, the Depository will withhold 31% of all
payments made prior to the time a properly certified TIN is provided to the
Depository. However, such amounts will be refunded to such holder if a TIN is
provided to the Depository within 60 days.
 
    NOTE: FAILURE TO COMPLETE AND RETURN THE SUBSTITUTE FORM W-9 MAY RESULT IN
BACKUP WITHHOLDING OF 31% OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER.
PLEASE REVIEW THE ENCLOSED GUIDELINES FOR CERTIFICATION OF TAXPAYER
IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL DETAILS.
 
    9. REQUESTS FOR ASSISTANCE OR ADDITIONAL COPIES.  Requests for assistance or
additional copies of the Offer to Purchase and this Letter of Transmittal may be
obtained from the Information Agent or Dealer Manager at their respective
addresses or telephone numbers set forth below.
 
    10. MYCOGEN CORPORATION 1992 STOCK OPTION PLAN.  Holders of options (each an
"Option") to purchase Shares through the Mycogen Corporation 1992 Stock Option
Plan (which incorporates outstanding Options granted under the Mycogen
Corporation 1983 Stock Option Plan) may not use this Letter of Transmittal to
direct the exercise of Options and the tender of Shares issuable upon exercise
of such Options, but must use the separate Option Election sent or delivered to
them by the Company. See Section 3 of the Offer to Purchase.
 
                                       6

    11. MYCOGEN CORPORATION 1995 EMPLOYEE STOCK PURCHASE PLAN.  Participants in
the Mycogen Corporation 1995 Employee Stock Purchase Plan (the "Stock Purchase
Plan") must use this Letter of Transmittal to direct the exercise of purchase
rights outstanding under the Stock Purchase Plan (each a "Purchase Right") and
the tender of Shares issuable upon exercise of such Purchase Rights, but must
use the separate Stock Purchase Election sent or delivered to them by the
Company. See Section 3 of the Offer to Purchase.
 
    12. MYCOGEN CORPORATION RESTRICTED STOCK ISSUANCE PLAN.  Holders of unvested
Shares ("Restricted Stock") through the Mycogen Corporation Restricted Stock
Issuance Plan MUST use this Letter of Transmittal to tender Restricted Stock,
and also MUST use the separate Restricted Stock Election sent or delivered to
them by the Company. See Section 3 of the Offer to Purchase. Completed Letters
of Transmittal relating to Restricted Stock should be returned, along with a
completed Restricted Stock Election and your Restricted Stock Certificate(s), to
Mycogen Corporation, AgroSciences Tender Offer, 5501 Oberlin Drive, San Diego,
California 92121, Attention: Cheri Manis. DO NOT return a Letter of Transmittal
relating to Restricted Stock to the Depositary.
 
    13. LOST, DESTROYED OR STOLEN CERTIFICATES.  If any certificate(s)
representing Shares has been lost, destroyed or stolen, the stockholder should
promptly notify the Depositary. Instructions will then be given to what steps
must be taken to obtain a replacement certificate(s). The Letter of Transmittal
and related documents cannot be processed until the procedures for replacing
such missing certificate(s) have been followed.
 
                                       7

    Facsimile copies of the Letter of Transmittal, properly completed and duly
executed, will be accepted. The Letter of Transmittal, certificates of Shares
and any other required documents should be sent or delivered by each stockholder
of the Company or his broker, dealer, commercial bank, trust company or other
nominee to the Depositary at one of its addresses set forth below:
 
                        THE DEPOSITARY FOR THE OFFER IS:
 
                                BANKBOSTON, N.A.
 

                                                         
           BY MAIL:                       BY HAND:              BY OVERNIGHT, CERTIFIED OR
       BankBoston, N.A.             Securities Transfer &         EXPRESS MAIL DELIVERY:
Attn: Corporate Reorganization    Reporting Services, Inc.           BankBoston, N.A.
        P.O. Box 8029              c/o Boston EquiServe LP           Attn: Corporate
    Boston, Massachusetts             1 Exchange Plaza                Reorganization
          02266-8029               55 Broadway, 3rd Floor           150 Royall Street
                                  New York, New York 10006     Canton, Massachusetts 02021

 

                           
       BY FACSIMILE:              CONFIRM FACSIMILE BY
    (781) 575-2233/2232                TELEPHONE:
 (For Eligible Institutions          (800) 730-4001
           Only)                 (For Confirmation Only)

 
    Questions and requests for assistance may be directed to the Information
Agent or the Dealer Manager at their respective addresses and telephone numbers
listed below. Additional copies of this Offer to Purchase, the Letter of
Transmittal and other tender offer materials may be obtained from the
Information Agent as set forth below, and will be furnished promptly at
Purchaser's expense. You may also contact your broker, dealer, commercial bank,
trust company or other nominee for assistance concerning this Offer.
 
                    THE INFORMATION AGENT FOR THE OFFER IS:
 
                                     [LOGO]
 
                               Wall Street Plaza
                            New York, New York 10005
                Call Collect (Banks and Brokers): (212) 440-9800
                   All Others Call Toll-Free: (800) 223-2064
 
                      THE DEALER MANAGER FOR THE OFFER IS:
 
                           SALOMON SMITH BARNEY INC.
                            Seven World Trade Center
                               New York, NY 10048
 
                                       8

- ------------------------------------------------
 
                          SPECIAL PAYMENT INSTRUCTIONS
                        (SEE INSTRUCTIONS 1, 5, 6 AND 7)
 
      To be completed ONLY if the check for the purchase price of Shares
  purchased or stock certificates for Shares not tendered or not purchased are
  to be issued in the name of someone other than the undersigned.
 
  Issue check or certificates to:
 
  Name: ______________________________________________________________________
                                 (PLEASE PRINT)
 
  Address ____________________________________________________________________
  ____________________________________________________________________________
                               (INCLUDE ZIP CODE)
  ____________________________________________________________________________
                  (TAX IDENTIFICATION OR SOCIAL SECURITY NO.)
                         (COMPLETE SUBSTITUTE FORM W-9)
 
- ------------------------------------------------
- ------------------------------------------------
 
                         SPECIAL DELIVERY INSTRUCTIONS
                        (SEE INSTRUCTIONS 1, 5, 6 AND 7)
 
      To be completed ONLY if the check for the purchase price of Shares
  purchased or stock certificates for Shares not tendered or not purchased are
  to be mailed to someone other than the undersigned or to the undersigned at
  an address other than that shown below the undersigned's signature(s).
  Mail check or certificates to:
  Name _______________________________________________________________________
                                 (PLEASE PRINT)
  Address ____________________________________________________________________
  ____________________________________________________________________________
                               (INCLUDE ZIP CODE)
- -----------------------------------------------
 
- --------------------------------------------------------------------------------
 
                                   SIGN HERE
                  (PLEASE COMPLETE SUBSTITUTE FORM W-9 BELOW)
 
  X___________________________________________________________________________
 
  X___________________________________________________________________________
                            SIGNATURE(S) OF OWNER(S)
 
  Dated: _______________________________________________________________, 1998
 
  Name(s) ____________________________________________________________________
                                 (PLEASE PRINT)
  ____________________________________________________________________________
  Capacity (full title): _____________________________________________________
  Address: ___________________________________________________________________
  ____________________________________________________________________________
                               (INCLUDE ZIP CODE)
  Area Code and Telephone Number: ____________________________________________
  Tax Identification or Social Security No.: _________________________________
                                         (COMPLETE SUBSTITUTE W-9 ON REVERSE
                                     SIDE)
 
      (Must be signed by registered holder(s) exactly as name(s) appear(s) on
  stock certificate(s) or on a security position listing or by person(s)
  authorized to become registered holder(s) by certificates and documents
  transmitted herewith. If signature is by a trustee, executor, administrator,
  guardian, attorney-in-fact, officer of a corporation or other person acting
  in a fiduciary or representative capacity, please set forth full title and
  see Instruction 5.)
 
                           GUARANTEE OF SIGNATURE(S)
                           (SEE INSTRUCTIONS 1 AND 5)
 
  Name of Firm: ______________________________________________________________
 
  Authorized Signature: ______________________________________________________
 
  Name: ______________________________________________________________________
 
  Address: ___________________________________________________________________
  ____________________________________________________________________________
                               (INCLUDE ZIP CODE)
 
  Area Code and Telephone Number: ____________________________________________
 
  Dated: _______________________________________________________________, 1998
 
- --------------------------------------------------------------------------------
 
                                       9

                 TO BE COMPLETED BY ALL TENDERING STOCKHOLDERS
                              (SEE INSTRUCTION 8)
 
                         PAYER'S NAME: BANKBOSTON, N.A.
 

                                                   
- -----------------------------------------------------------------------------------------
           SUBSTITUTE             Part I--PLEASE              Social Security Number
            FORM W-9              PROVIDE YOUR TIN IN      or ------------------------
   Department of the Treasury     THE BOX AT RIGHT AND    Employer Identification Number
    Internal Revenue Service      CERTIFY BY SIGNING
                                  AND DATING BELOW
- -----------------------------------------------------------------------------------------
 CERTIFICATION--Under penalty of perjury, I certify that:
 
 (1) The number shown on this form is my correct taxpayer identification number (or I am
     waiting for a number to be issued to me);
 
 (2) I am not subject to backup withholding because (a) I am exempt from backup
     withholding, or (b) I have not been notified by the Internal Revenue Service ("IRS")
     that I am subject to backup withholding as a result of a failure to report all
     interest or dividends, or (c) the IRS has notified me that I am no longer subject to
     backup withholding.
 
 CERTIFICATION INSTRUCTIONS--You must cross out item (2) above if you have been notified
 by the IRS that you are currently subject to backup withholding because of
 underreporting or dividends on your tax return. However, if after being notified by the
 IRS that you were subject to backup withholding you received another notification from
 the IRS that you are no longer subject to backup withholding, do not cross out such item
 (2).
 
 SIGNATURE: -------------------------------------------- DATE:--------------------, 1998
- -----------------------------------------------------------------------------------------

 
NOTE: FAILURE TO COMPLETE AND RETURN THIS FORM MAY RESULT IN BACKUP WITHHOLDING
      OF 31% OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW
      THE ENCLOSED GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION
      NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL DETAILS.
 
YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU ARE AWAITING A TAX
IDENTIFICATION NUMBER.
 


- ----------------------------------------------------------------------------------------
                 CERTIFICATE OF AWAITING TAXPAYER IDENTIFICATION NUMBER
                                                                                        
    I certify under penalties of perjury that a taxpayer identification number has not
been issued to me, and either (1) I have mailed or delivered an application to receive a
taxpayer identification number to the appropriate Internal Revenue Service Center or
Social Security Administration Office or (2) I intend to mail or deliver an application
in the near future. I understand that if I do not provide a taxpayer identification
number by the time of payment, 31% of all reportable payments made to me will be
withheld, but that such amounts will be refunded to me if I then provide a Taxpayer
Identification Number within sixty (60) days.
SIGNATURE: ------------------------------------------- DATE: -------------------, 1998
- ----------------------------------------------------------------------------------------

 
                                       10