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June 9, 1998 (REVISION)

Thomas G. Klopack
707 Linden Avenue
Los Altos, CA  94022

Dear Tom:

This letter is a formal offer setting forth the principal terms for you to 
join Aurora Biosciences Corporation (the "Company"), a Delaware corporation, 
which is located in San Diego, California.

POSITION:              Senior Vice President, Strategic Operations

REPORTING TO:          President

BASE SALARY RATE:      $15,000.00 per month

HIRING BONUS:          $20,000.00, less normal withholdings, payable with your
                       first paycheck.  If you leave voluntarily before the end
                       of the first twelve months of employment, you agree to
                       repay this amount to Aurora Biosciences in full.

1998 PERFORMANCE
BONUS:                 You will be guaranteed a year-end bonus in the amount of
                       $10,000 assuming your performance meets position
                       expectations. This bonus will be paid at the end of
                       1998 or the first pay period of 1999. (If your
                       board-approved bonus for 1998 would have been larger than
                       $10,000.00, you will receive this larger amount.)

EQUITY:                Upon commencement of your employment with the Company,
                       you will be granted an Option to purchase 100,000 shares
                       of the Common Stock of the Company. The exercise price
                       per share of the Option is the closing sales price for
                       the stock on the last market trading day prior to your
                       date of employment as reported in the Wall Street
                       Journal.

                       The shares of Common Stock subject to your Option will
                       vest according to the following schedule: 25% will vest
                       on the first anniversary of your start date; the
                       remainder of the shares will vest monthly thereafter over
                       the following three year period at the rate of one
                       forty-eighth (1/48) of such shares each month.

                       The specific terms and conditions of your Option to
                       purchase shares of the Common Stock of the Company will
                       be set forth in an agreement between you and the Company.
                       This Agreement will be executed after you commence your
                       employment with the Company.
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RELOCATION:            Reimbursed expenses to include reasonable and customary
                       closing costs for the sale of your house, including
                       realtors' fees (to a maximum of $35,000); or the
                       documented expenses associated with renting your current
                       home to a maximum of $7,500.00 and a housing allowance
                       of up to $5,000.00 per month to a maximum of $16,000.00
                       for your family's accommodations in San Diego in the
                       six-month period following your start date.  In addition,
                       the Company will pay for the movement of your household
                       goods, including packing, unpacking, and insurance to San
                       Diego. Payment for the move of up to two vehicles will
                       also be included. All expenses must be documented and
                       paid either directly to the vendor or reimbursed by
                       normal means. The Company will follow federal, state and
                       local tax regulations with regards to reporting
                       reimbursements associated with the move. This allowance
                       will be "grossed up" to minimize federal/state income tax
                       impact on you. The terms of this move package are valid
                       for 12 months from your date of employment. You agree to
                       repay all relocation expenses if you terminate employment
                       voluntarily within 12 months of the date the expenses are
                       submitted.

TEMPORARY HOUSING:     You will be reimbursed for temporary housing expenses for
                       sixty days to a maximum of $5,000. You will need to
                       furnish receipts documenting actual expenses incurred.

HOME PURCHASE LOAN:    A loan in the amount of $60,000, secured by the primary
                       residence you purchase in San Diego County, will be made
                       to you by Aurora interest free.  The loan will be
                       forgiven annually over four years beginning December 31,
                       1999 through December 31, 2002, at the rate of $15,000
                       per year. For each of the calendar years 1999, 2000,
                       2001, and 2002, any performance-based bonus that you
                       might receive under Aurora's Performance Bonus Plan will
                       be reduced by 50%, up to a maximum reduction of $10,000
                       for that Plan Year, to reduce the forgivable portion of
                       the housing loan.  You must be employed by Aurora on
                       December 31 of any of these years to have the forgivable
                       portion forgiven in that year.  Any unforgiven loan
                       balance will be payable within six months following
                       termination of your employment for any reason. The loan
                       will be evidenced by a promissory note to be executed by
                       you at the time of escrow closing of the San Diego
                       property.  Such note would contain the terms set forth
                       herein, as well as other terms customary for such a loan.

BENEFITS:              You will be entitled to receive standard medical, life
                       and dental insurance benefits for yourself and your
                       dependents in accordance with Company policy.

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TRAVEL:                You will be eligible for airfares for you and your family
                       members for the move to San Diego plus one additional
                       house hunting trip, expenses for such house hunting trip
                       not to exceed $2,000, for you and your family.

SEVERANCE:             You will be entitled to a cash severance payment equal to
                       twelve months' salary if your employment is terminated by
                       the Company for other than for cause within 12 months of
                       your employment, or equal to nine months pay if your
                       employment is terminated other than for cause between
                       twelve and eighteen months after your start date, or
                       equal to six months pay if your employment is terminated
                       other than for cause between eighteen and twenty four
                       months after your start date.

BENEFITS:              You will be entitled to receive standard medical, life
                       and dental insurance benefits for yourself and your
                       dependents in accordance with Company policy.

401(K) PLAN:           You will be eligible to participate in the Aurora
                       Biosciences Corporation 401(K) Savings Plan on the first
                       of the month following your employment.  You may
                       contribute up to 18% of your earnings or $10,000
                       whichever is less.  Aurora contributes $0.50 on the
                       dollar for the first 4% of your earnings contributed with
                       a maximum matching contribution of $1,600 per year.  This
                       matching contribution is vested over four years of
                       service at Aurora.

PAID PERSONAL LEAVE:   Although our policy provides 17 days of Paid Personal 
                       Leave (PPL), you will accrue at the rate of 20 days per 
                       year during your first year of service. At each 
                       subsequent anniversary of your employment, an additional 
                       day will be added to the accrual rate to a maximum 
                       accrual of 25 days. If Aurora elects to close operations 
                       between Christmas and New Year's, this time off will be 
                       considered paid holiday time and will not count against 
                       paid personal leave.

EMPLOYMENT AT WILL:    Your employment will be at will, which means it may be
                       terminated at any time by you or the Company with or
                       without cause.

START DATE:            July 13, 1998

As a condition of your employment, you will be required to sign a copy of our 
Employee Proprietary Information and Inventions Agreement when you begin your 
employment. In addition, to conform with the Immigration Reform and Control 
Act of 1986, you will be required to provide sufficient documentation to show 
proof of employment eligibility in the United States. PLEASE BRING WITH YOU 
ON YOUR START DATE, THE ORIGINAL OF ONE OF THE DOCUMENTS NOTED IN LIST A OR 
ONE DOCUMENT FROM LIST B AND ONE DOCUMENT FROM LIST C as itemized in the 
enclosed "Lists of Acceptable Documents". If you do not have the originals of 
any of these documents, please contact me immediately.

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It is Aurora's policy to respect fully the rights of your previous employers 
in their proprietary or confidential information. No employee is expected to 
disclose, or is allowed to use for Aurora's purposes, any confidential or 
proprietary information he or she may have acquired as a result of previous 
employment.

I am pleased to extend this offer to you and look forward to your acceptance. 
Please sign and return the enclosed copy of this offer letter as soon as 
possible to indicate your agreement with the terms of this offer. This offer 
will lapse if not signed and returned by fax to 619-452-9940 by June 20, 1998.

Once signed by you, this letter will constitute the complete agreement 
between you and Aurora regarding employment matters and will supersede all 
prior written or oral agreements or understandings on these matters.

I believe you will be able to make an immediate contribution to Aurora's 
effort, and I think you will enjoy the rewards of working for an innovative, 
fast-paced company. One of the keys to our accomplishments is good people. We 
hope you accept our offer to be one of those people.

Yours sincerely,

/s/ TIMOTHY J. RINK

Timothy J. Rink, M.D., Sc.D
Chairman, CEO & President

TJR/pf
Enclosures

I ACCEPT THE TERMS OF EMPLOYMENT AS DESCRIBED IN THIS OFFER LETTER DATED JUNE 
9, 1998 AND WILL START MY EMPLOYMENT ON JULY 13, 1998. I CONFIRM THAT BY MY 
START DATE AT AURORA I WILL BE UNDER NO CONTRACT OR AGREEMENT WITH ANY OTHER 
ENTITY WHICH WOULD IN ANY WAY RESTRICT MY ABILITY TO WORK AT AURORA OR 
PERFORM THE FUNCTIONS OF MY JOB FOR AURORA, INCLUDING, BUT NOT LIMITED TO, 
ANY EMPLOYMENT AGREEMENT AND/OR NON-COMPETE AGREEMENT.

/s/ THOMAS G. KLOPACK                           DATE
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   THOMAS G. KLOPACK