<PAGE>   1
                                                                   EXHIBIT 10.20

                              EMPLOYMENT AGREEMENT

        THIS AGREEMENT made as of the date last written below by and between
Genencor International, Inc. ("GCI"), a Delaware Corporation with its office at
Four Cambridge Place, 1870 South Winton Road, Rochester, New York 14618 and W.
Thomas Mitchell, ("Employee"), residing at 9 Bridgewater Court, Pittsford, New
York 14534.

        WHEREAS, GCI desires to employ the Employee and the Employee desires to
work for GCI, and GCI and the Employee desire to define the terms and conditions
under which GCI will employ the Employee.

        NOW, THEREFORE, the parties hereby covenant and agree as follows:

        1. GCI hereby employs the Employee as Chief Executive Officer/President
to perform such duties consistent with his title and position as may be
determined and assigned to him by the Board of Directors of GCI.

        2. The Employee agrees to devote substantially all of his professional
employment time and effort to the performance of his duties as such Chief
Executive Officer/President for GCI and to perform such other duties consistent
with his title and position as are reasonably assigned him from time to time by
the Board of Directors of GCI.

        3. Unless terminated (1) by either GCI or Employee in writing, as
hereinafter specifically provided, or (2) unless either party objects to the
continuation of this agreement in writing, the initial term of this Agreement
shall be two (2) years from the last

<PAGE>   2
                                      -2-


date written below, and will be automatically renewed for incremental 2-year
periods thereafter, unless agreed to in writing by both parties. A review of the
Employee's total compensation based on GCI's assessment of Employee's
contribution to GCI's performance will be conducted each year following
submission of GCI's audited financial results to the Board of Directors of GCI.

        4. For all the services to be rendered by the Employee in any capacity
hereunder, including services as an officer, director, member of any committee
or any other duties assigned him by the Board of Directors of GCI, GCI agrees to
pay the Employee a salary of $281,000 Dollars per annum, payable in accordance
with the customary payroll payment practices of GCI. The foregoing annual
compensation amount may be, from time to time, increased by action of the
Company's Board of Directors or the appropriate Committee of such Board. Such
action with respect to annual compensation shall constitute an amendment to this
Agreement. It is understood and agreed that the granting of a cash bonus for
performance in any given year shall not constitute an amendment to this
Agreement.

        5. GCI and the Employee hereby agree that nothing contained herein is
intended to or shall be deemed to affect any of the Employee's rights as a
participant under any retirement, stock option, stock purchase, pension,
insurance, profit-sharing, bonus or similar plans of GCI now or hereafter
declared to be in effect. GCI recognizes that the Employee is induced to execute
this Agreement and to accept compensation at the rate set forth herein in part
because he expects to be a participant under such plans as are, from time to
time, in effect for the Company's executives

<PAGE>   3
                                      -3-


and/or employees in general.

        6. The Employee agrees to execute and be bound by an Employee
Confidentiality, Non-Disclosure, Non-Competition Agreement in the form attached
hereto as Exhibit A; the Invention Disclosure/Assignment Agreement attached as
Exhibit B; and Form of Confidentiality Agreement attached as Exhibit C; and
whether employed by GCI or not, agrees to execute Exhibit C at any time at the
direction of the Board of Directors of GCI in order to avoid disclosure of
confidential information [as defined in Exhibit A, paragraph (1)] as this
Exhibit is needed at a future date. The terms of each Exhibit are hereby
incorporated by reference and made a part hereof.

        7. This Agreement may be terminated by the Board of Directors at any
time before the expiration of the term provided for herein if, during the term
of this Agreement, the Employee (a) materially violates the provisions of
Exhibits A and/or B as executed (exhibits incorporated by reference herein); or
(b) refuses to execute Exhibit C as directed, (exhibit incorporated by reference
herein); (c) is convicted in a court of law of a felony or any crime involving
misuse or misappropriation of money or other property of GCI; (d) exhibits
repeated willful or wanton failure or refusal to perform his duties in
furtherance of GCI's business interest or in accordance with this Agreement
which failure or refusal is not remedied by the Employee within thirty (30) days
after written notice from GCI's Board of Directors; (e) commits an intentional
tort against GCI; (f) commits any flagrant act of dishonesty or disloyalty or
any act involving gross moral turpitude

<PAGE>   4
                                      -4-


which materially adversely affects the business of GCI; or (g) exhibits
immoderate use of alcohol or drugs which, in the opinion of an independent
physician, impairs the Employee's ability to perform his duties hereunder (all
of the foregoing clauses (a) through (g) constituting reasons for termination
"for cause") provided that unsatisfactory business performance of GCI, or mere
inefficiency, or good faith errors in judgment or discretion by the Employee
shall not constitute grounds for termination for cause hereunder. In the event
of such termination for cause, GCI may on ten (10) days' notice then terminate
his employment and, in that event, GCI shall be obligated only to pay the
Employee the compensation due him up to the date of termination, all accrued,
vested or earned benefits under any applicable benefit plan and any other
compensation to which the Employee is entitled under this agreement up to and
ending on the date of the Employee's termination. The benefits and compensation
outlined above are the only compensation and benefits which the Employee will
receive if the Employee voluntarily terminates his employment. Employee agrees
to give GCI three (3) months' notice prior to resignation, voluntarily quitting,
etc., and in exchange GCI agrees to pay Employee for this period.

        8. Except for termination for cause as defined in paragraph 7 (or
voluntary termination by the Employee), in the event the Employee is terminated
without cause by GCI at any time prior to the end of the original term of the
Agreement, or prior to the end of any incremental renewal period, for any other
reason(s), including but not limited to a change in the ownership or control

<PAGE>   5
                                      -5-


of GCI, such as may occur through divestiture, merger, acquisitions,
consolidation or takeover, or due to action of the Board of Directors, or due to
a substantial reduction in the duties to be performed by the Employee resulting
in actual or constructive removal from the position held by the Employee on the
effective date of this Agreement, or due to substantial change in the financial
conditions of GCI as evidenced by GCI filing a petition for reorganization under
any bankruptcy, insolvency, reorganization or similar law or making an
assignment for the benefit of creditors, then the Employee shall, upon such
termination, receive Termination Compensation, including (a) salary at the
highest annual salary rate (excluding non-recurring bonuses) paid the Employee
during any previous annual period of employment payable for twenty-four (24)
months unless the Employee becomes employed by a competitor as defined in
Exhibit A (page 2) during that period lasting up to 24 months; and (b)
continuation of all company-paid benefits or additional compensation sufficient
for the Employee to acquire equivalent benefits, for the same twenty-four month
period; and (c) customary outplacement services limited to $12,000.

        Notwithstanding the foregoing, GCI shall be entitled, by providing
notice in writing to the Employee, to terminate his employment under this
Agreement if the Employee shall become permanently disabled such that he is
unable to carry out his duties hereunder for three (3) consecutive calendar
months or for a period aggregating one hundred twenty (120) days in any period
of twelve (12) consecutive calendar months. If the Employee is approved to
receive benefits under GCI's LTD Plan, then GCI will pay the

<PAGE>   6
                                      -6-

Employee additional compensation so that the total equals the Termination
Compensation described in this paragraph 8 for the twenty-four (24) month term.
If the Employee is not approved to receive benefits under GCI's LTD Plan, then
he will, upon termination of his employment, be entitled to receive the full
Termination Compensation as described in this paragraph 8. Any delay or
forbearance by the Company in exercising any such right to terminate this
Agreement shall not constitute a waiver thereof.

        All Termination Compensation paid, if any, shall be payable in
accordance with the customary payroll practices of GCI unless GCI and the
Employee agree that GCI shall make one lump-sum payment at a time and in an
amount agreeable to both. The Employee's entitlement to Termination Compensation
payment as provided herein shall be in addition to any rights the Employee may
have to payments or participation under any retirement, stock option, stock
purchase, pension, insurance, profit-sharing, bonus or similar plans applicable
to the Employee or employees in general, as defined in the appropriate Plan
Documents. In addition, if GCI provides notice that this Agreement is
terminated, GCI shall have no additional obligations hereunder, other than to
pay to the Employee (a) any unpaid amount of accrued salary (as defined) in
paragraph 4 on page 2 herein); (b) any unpaid amount of accrued vacation pay in
accordance with GCI policy; (c) a pro rata amount of any vested incentive
compensation that shall be awarded the Employee pursuant to GCI policy; and (d)
other obligations which may be owed the Employee under a specific provision of
this agreement.

<PAGE>   7
                                      -7-


        9. This Agreement shall be construed and performed in accordance to the
laws of the State of New York.

        10. All notices provided for or permitted to be given pursuant to this
Agreement must be in writing. All notices shall be personally delivered or sent
by registered or certified mail to GCI or the Employee at the address set forth
above or to such other address as GCI or the Employee may notify the other in
accordance with the provisions of this section, or the last known permanent
residence. Any such notice so sent by mail shall be deemed made or given upon
mailing.

        11. This Agreement contains the sole and entire agreement of the parties
and supersedes all prior agreements and understandings between the Employee and
GCI (including but not limited to the Agreement dated December 10, 1991) and
cannot be modified or changed by any oral or verbal promise or statement by
whomsoever made; nor shall any written modification of it be binding upon GCI
until such written modification shall have been approved in writing by at least
two members of the GCI Board of Directors, one representing each owner.

        12. In the event any term or condition contained in this Agreement
should be breached by any party and thereafter waived or consented to by the
other party, such waiver or consent shall be limited to the particular breach so
waived or consented to and shall not be deemed to waive or consent to any other
breach occurring prior or subsequent to the breach so waived or consented to.

<PAGE>   8
                                      -8-


        13. If any provisions of this Agreement or the application thereof to
any person or circumstances shall be invalid or unenforceable to any extent, the
remainder of this Agreement and the application of such provisions to other
persons or circumstances shall not be affected thereby and shall be enforced to
the extent permitted by law.

        14. The provisions hereof, including without limitation those
incorporated herein pursuant to Section 6, which are to be performed or observed
after the termination of this Agreement, and the representations, covenants and
agreements of the parties contained herein with respect thereto shall survive
the termination of this Agreement and be effective according to their terms.

        15. All of the terms and provisions of this Agreement shall be binding
upon and shall inure to the benefit of and be enforceable by and against the
parties to this Agreement and the respective heirs, executors, and successors in
interest; provided, however, that the duties of the Employee hereunder are
personal in nature and may not be delegated without a written consent of GCI.

        16. This Agreement, and the rights and benefits contained herein, may
not be assigned by either party hereto.

        17. The Employee shall be based in Rochester, New York. The Employee may
be transferred at any time and as many times as the Company shall determine,
upon six months' prior written notice to the Employee.

<PAGE>   9
                                      -9-


        IN WITNESS WHEREOF, GCI has caused this Agreement to be executed by the
Board of Directors of GCI, and the Employee has hereunto set his hand as of the
day and year first above written.


                               GENENCOR INTERNATIONAL, INC.

                               By: /s/ Bjorn Mattsson
                                  ------------------------------------
                                   Bjorn Mattsson
                                   Chairman

                               By: /s/ Jerry D. Holmes
                                  ------------------------------------
                                   Jerry D. Holmes
                                   Vice Chairman

/s/ W. Thomas Mitchell
------------------------------
W. Thomas Mitchell

Date:   6/21/95
     -------------------------

<PAGE>   10
                                    EXHIBIT A

                    EMPLOYEE CONFIDENTIALITY, NON-DISCLOSURE
                          AND NON-COMPETITION AGREEMENT


     THIS AGREEMENT is made as of the date last written below by and between
Genencor International, Inc. ("GCI"), a Delaware Corporation having its
principal offices at Four Cambridge Place, 1870 South Winton Road, Rochester,
New York 14618 and W. Thomas Mitchell ("Employee"), residing at 9 Bridgewater
Court, Pittsford, New York 14534.

     WHEREAS, the Employee is an employee of GCI who, during the course of such
employment, will be working upon and have access to certain confidential
information, processes, technical data, trade secrets, know-how and other
business information of a confidential nature belonging to GCI; and

     WHEREAS, GCI and the Employee wish to enter into certain covenants to
preserve and foster their respective business interests and, in certain
respects, their future cooperation with their fellow employees; and

     WHEREAS, employees similarly situated as the Employee are separately
covenanting and agreeing not to compete with GCI should they leave the
employment of GCI under certain defined circumstances; and

     THEREFORE, in consideration of the foregoing premises and the covenants
herein contained, and in consideration of GCI's employment of the Employee for
such period or periods as may from time to time be mutually agreed upon, and of
the wages or salary paid or agreed to be paid to the Employee, and other good
and valuable consideration, the adequacy of which is hereby

<PAGE>   11
                                      -2-


acknowledged, the parties hereto agree as follows:

     1. Confidentiality and Non-Disclosure and Non-Use. The Employee agrees and
covenants that he will not, at any time during his employment by GCI and for a
period of five (5) years after his employment without the prior written
authorization of GCI, disclose, or use on his own behalf or use with any
unauthorized person any formulas, methods, compositions, trade secrets, secret
processes, technical data, confidential business information or other know-how
or matters of a secret, proprietary or confidential nature relating to GCI or
its business which the Employee gained access to or knowledge of during or by
reason of his employment with GCI. The foregoing obligations regarding
non-disclosure and non-use shall not apply to information which: (a) was in the
Employee's possession before receipt from GCI; or (b) is or becomes a matter of
public knowledge through no fault of the Employee; (c) is disclosed by GCI to a
third party without duty of confidentiality on the third party; or (d) is
disclosed under operation of law, if possible in conformity with Exhibit C.

     2. Non-competition.

        A. In the sole event of the Employee's voluntary withdrawal from GCI's
employment or Termination of Employment for cause as defined in paragraph 7 of
the Employment Agreement to which this Exhibit A is appended, until the
expiration of a 24 month period commencing on the date of the termination of his
employment, the Employee shall not engage or compete directly or indirectly, as
a principal, on his own account, or as a shareholder in, or employee of, any
corporation or legal entity selling, manufacturing or

<PAGE>   12
                                      -3-


developing products in the field of Industrial Enzymes which are or may be
competitive with those marketed or being developed by GCI on the date of the
termination of his employment. (If the Employee is unaware of GCI's development
of any projects at the time of the termination of his employment, GCI retains
the discretion as to whether to inform the Employee of products it is then
developing or considering to develop, but if GCI chooses not to inform the
Employee of the development of any project currently being pursued or considered
by GCI, all of which are unknown by the Employee, this will not bar the Employee
from obtaining employment with a competitor.) The foregoing non-compete
restrictions shall likewise apply to employment or competition in any other
field of business in which, on the date of employment termination, GCI is
manufacturing or selling products in commerce, or for use in commerce, in excess
of $10.0 Million U.S. Dollars annually or has committed to commercialize with
internal resource expenditures in excess of $5.0 Million U.S. Dollars or has
signed a binding contract with a third party concerning such other field of
business in which the value to be received by GCI is in excess of $5.0 Million
U.S. Dollars. The Employee, further, shall not lend credit of money for the
purpose of establishing or operating any such business, nor furnish any
information (including the information subject to the restriction in paragraph l
above) or give advice, either directly or indirectly, to any such third party,
corporation or business entity of any kind. The non-compete restrictions of this
paragraph (2A) shall apply, in the case of a large corporation conducting
business in diverse business fields, only to employment

<PAGE>   13
                                      -4-


or competition in that unit, division, subsidiary or other part of such
corporation (or other legal entity) in competition with GCI in the fields
defined herein and shall not preclude the Employee from being engaged or
employed by such corporation in other non-competitive fields as provided for in
paragraph 2.C.

        If the Employee is terminated without cause, he will receive Termination
Compensation as defined by paragraph 8 of the Employment Agreement unless he
becomes employed by a competitor as previously defined herein. At that time, all
compensation from GCI ceases.

        For a period of twenty-four (24) months following the Employee's
termination from GCI, the Employee agrees to disclose the name of any employer
to the Chairman of the Board of Directors two weeks in advance of his employment
with any competitor, non-competitor, business or his embarking upon
self-employment or consulting.

        B. It is recognized by GCI and the Employee that his efforts, and those
of his fellow employees are critically important to the overall profitability of
GCI. The future profitability of GCI is also linked to the continuing services
of the Employee and the covenant of the Employee not to compete with GCI should
he choose to leave the employ of GCI.

        C. The foregoing covenant and agreement is not intended to and shall not
prevent the Employee from seeking or accepting alternative employment with other
business entities, customers or suppliers of GCI so long as such business
entities, customers or suppliers are not in the business of selling,
manufacturing or

<PAGE>   14
                                      -5-


developing products in the field of Industrial Enzymes or products in other
field(s) of business as referred to above which are or may be competitive with
those being marketed, developed or proposed for development by GCI within two
(2) years after the date of the termination of the Employee's employment.

     3. Exception for Publicly-Traded Companies. The foregoing agreement not to
compete shall not prohibit the Employee from owning stock as an investment,
debentures, warrants or similar instruments in any company whose stock is traded
on a national securities exchange or over-the-counter so long as such ownership
interest is less than five percent (5%) of the outstanding and traded stock,
debentures or warrants, as the case may be.

     4. Equitable and Legal Remedies. The parties hereto agree that no remedy of
law will be adequate to compensate GCI for a violation of this Agreement; and
the Employee hereby agrees that in addition to any legal or other rights that
may be available in the event of a breach hereunder, GCI may seek equitable
relief to enforce this Agreement in any court of competent jurisdiction. Any
actions or proceedings brought regarding this Agreement shall be venued in the
Western District of New York or in the County of Monroe, State of New York.



     5. Miscellaneous

         A. This Agreement and all rights and obligations hereunder shall be
governed and construed in accordance with the laws of the State of New York.

         B. This Agreement shall be binding upon and inure to the benefit of the
parties hereto and their respective legal

<PAGE>   15
                                      -6-


representatives, successor and assigns.

        C. This Agreement shall not be modified, amended, assigned, cancelled or
superseded except in writing signed by GCI and the Employee.

        D. This Agreement contains the entire agreement between the parties
relating to the subject matter hereof and supersedes and cancels all prior
proposals and understandings, whether written or oral, relating to the subject
matter hereof.

        E. No failure on the part of GCI or of the Employee to exercise, and no
delay in exercising any right or remedy hereunder shall operate as a waiver
thereof or of any other right or remedy; nor shall any single or partial
exercise thereof preclude any other or further exercise thereof or of any right
or remedy.

        F. If any provision of this Agreement shall be prohibited by or be
invalid under applicable law, such provision shall be ineffective only to the
extent of such prohibition or invalidity without invalidating the remainder of
such provision or the remaining provisions of this Agreement.

        G. All notices, requests and demands shall be in writing by certified
mail, return receipt requested, addressed to the respective parties hereto at
their respective addresses first hereinabove set forth or to such other address
as either party shall designate in a written notice similarly given to the other
party.


<PAGE>   16



           IN WITNESS WHEREOF, the parties have executed this Agreement on the
date first above written.

                               GENENCOR INTERNATIONAL, INC.

                               By: /s/ Bjorn Mattsson
                                   -------------------------------
                                   Bjorn Mattsson
                                   Chairman

                               By: /s/ Jerry D. Holmes
                                   -------------------------------
                                   Jerry D. Holmes
                                   Vice Chairman

/s/ W. Thomas Mitchell
-----------------------------------
W. Thomas Mitchell

Date:   6/21/95
     ------------------------------

<PAGE>   17

                                    EXHIBIT B

                    INVENTION DISCLOSURE/ASSIGNMENT AGREEMENT



        It is my understanding that Genencor International, Inc., (hereinafter
called "GCI") is engaged in the business of industrial biotechnology, including
research, development and manufacturing. I also understand that, as is generally
customary, GCI requires its employees to assign to it all rights, title and
interest in and to all inventions, discoveries, improvements and copyrightable
subject matter (hereinafter separate or collectively called "rights") in the
field of employment of its various employees and that it is essential for the
full protection of the business of GCI that employees shall not disclose
classified or confidential or proprietary Company information regarding such
business, with which information they have or may become acquainted during the
period of their employment.

        Therefore, in consideration of my employment or continued employment by
GCI during such time as I may be employed by GCI, and of the wages or salary and
other benefits to be received by me in respect to such employment, it is
understood and agreed as follows:

        I hereby do and will sell, assign and transfer to GCI all of my rights,
title and interest in and to all said rights which, during, or within two years
after the termination of, my employment by GCI, have been or may be made or
conceived by me, alone or with others, and within or arising out of any field of
employment in which I have worked or shall work for GCI or arising out of any
information regarding the business of GCI which has been or may be received by
me while in GCI's employment.

<PAGE>   18
                                      -2-


        I will fully disclose to GCI as promptly as available all information
known or possessed by me concerning the rights mentioned in the preceding
paragraph; and, upon request of GCI and without further remuneration to me by
GCI, but at the expense of GCI, I will execute all applications for patents and
for copyright registration, assignments thereof and other instruments, and do
all things which GCI may deem necessary to vest and maintain in it my entire
right, title and interest in and to all such rights.

        This agreement replaces all previous agreements relating to the same or
similar matters which I may have entered into with GCI with respect to my
present and future period of employment by GCI. It shall inure to the benefit of
the successors and assigns of GCI, and shall be binding upon my heirs, assigns,
administrators and representatives. No oral agreement, statement or
representation shall alter the provisions of this agreement.




/s/ W. Thomas Mitchell
-----------------------------
W. Thomas Mitchell


Date:  June 21, 1995
     ------------------------

<PAGE>   19

                                    EXHIBIT C

                        FORM OF CONFIDENTIALITY AGREEMENT



        Confidentiality Agreement between Genencor International, Inc., a
corporation incorporated and existing under the laws of Delaware and having its
office in Rochester, New York, (hereinafter referred to as "GCI") and W. Thomas
Mitchell hereinafter referred to as "Employee").

        WHEREAS the Employee desires to obtain or elicit confidential testimony
or documents from GCI for the purpose of litigation or arbitration between or
among W. Thomas Mitchell and GCI who desires to maintain the confidentiality of
said testimony or documents.

        THEREFORE, in consideration of the foregoing and of the mutual promises
hereinafter set forth, and of other good and valuable considerations the receipt
and sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

        1. All testimony or documents generated, produced, referred to, or
elicited in the course of the litigation or arbitration may, as appropriate, be
designated by either GCI or the Employee to be confidential ("Confidential
Material"). The party producing a document may designate it as Confidential
Material by marking it with the notation "CONFIDENTIAL." The party may designate
testimony as Confidential Information at the time of the deposition or testimony
of a representative of a party. Testimony so designated shall be transcribed
separately from the rest of the deposition. Alternatively, a party may designate
testimony as Confidential Information by written advice to all parties of the
pages and lines of the transcript so designated, within thirty (30)

<PAGE>   20
                                      -2-


days of the receipt by counsel of such transcript, in which event the portion
designated confidential shall be removed from the transcript and transcribed
separately. During said thirty (30) day period, all testimony shall be deemed
and treated as Confidential Information unless otherwise instructed by
designating counsel or the court or arbitration panel.

        2. Confidential Material and any information derived therefrom which
tends to reveal the contents of Confidential Material may be inspected or used
only for the purposes of this action and only by: (a) the attorneys for the
parties and persons regularly employed by them; (b) the parties; (c) any person
employed to assist the aforementioned persons in connection with the preparation
and trial or arbitration of this action and any appellate or judicial review;
and (d) the respective court or arbitration panel before which this action is
pending, court or arbitration employees, court reporters, stenographic reporters
and members of the jury or arbitration panel. No other person shall have access
to Confidential Material or be informed of the contents thereof. If appellate or
judicial review is pursued, Confidential material may be included in the record
under appeal or review, but is, if at all possible, to be "sealed," marked
CONFIDENTIAL, and not made available for public review.

        3. Confidential Material in the form of testimony or documents,
including any copies thereof, shall be returned to the party who produced or
generated them either within 30 days following the completion of the trial or
arbitration, or, if appellate or judicial review is pursued, within 30 days
following

<PAGE>   21
                                      -3-


entry of a final order dispositive of the matter and the time for any further
appeal has expired or the order is otherwise unappealable.

Dated:
      ---------------------------
                                             GENENCOR INTERNATIONAL, INC.

                                             By:
                                                ---------------------------

                                             Name:
                                                  -------------------------

                                             Title:
                                                   ------------------------

-----------------------------
W. Thomas Mitchell