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March 13, 2000

Gregory Sessler
9011 N.E. 34th Street
Bellevue, Washington 98004

Dear Greg:

         On behalf of ROSETTA INPHARMATICS, Inc. (the "COMPANY"), I am pleased
to offer you the position of Senior Vice President, Chief Financial Officer.
Speaking for myself, as well as the other members of the Company's management
team, we are all very impressed with your credentials and we look forward to
your future success in this position.

         The terms of your new position with the Company are as set forth below:

         1.       POSITION.

                  a.       You will become a Senior Vice President, Chief
Financial Officer working at the Company's Kirkland office, under the direction
of Stephen H. Friend, M.D., Ph.D., Chief Executive Officer.

                  b.       You agree to the best of your ability and experience
that you will at all times loyally and conscientiously perform all of the duties
and obligations required of and from you pursuant to the express and implicit
terms hereof, and to the reasonable satisfaction of the Company. During the term
of your employment, you further agree that you will devote all of your business
time and attention to the business of the Company, after the part-time schedule
as described in section 2 below, the Company will be entitled to all of the
benefits and profits arising from or incident to all such work services and
advice, you will not render commercial or professional services of any nature to
any person or organization, whether or not for compensation, without the prior
written consent of the Company's Board of Directors, and you will not directly
or indirectly engage or participate in any business that is competitive in any
manner with the business of the Company. Nothing in this letter agreement will
prevent you from accepting speaking or presentation engagements in exchange for
honoraria or from serving on boards of charitable organizations, or from owning
no more than one percent (1%) of the outstanding equity securities of a
corporation whose stock is listed on a national stock exchange, except for
ownership in the stock of SONUS Pharmaceuticals, Inc.

         2.       START DATE. Subject to fulfillment of any conditions imposed
by this letter agreement, you will commence this position with the Company on or
before March 13, 2000. The Company understands that you will work a part-time
schedule, minimum of one day per week, initially, not to exceed six weeks.


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         3.       PROOF OF RIGHT TO WORK. For purposes of federal immigration
law, you will be required to provide to the Company documentary evidence of your
identity and eligibility for employment in the United States. Such documentation
must be provided within three business days of your date of hire, or your
employment may be terminated.

         4.       COMPENSATION.

                  a.       BASE SALARY. You will be paid a monthly salary of
$15,833, which is equivalent to $190,000 on an annualized basis. Your salary
will be payable in TWO EQUAL PAYMENTS PER MONTH pursuant to the Company's
regular payroll policy (or in the same manner as other employees of the
Company).

                  b.       ANNUAL REVIEW. Your base salary will be reviewed at
the end of each calendar year as part of the Company's normal salary review
process.

         5.       STOCK OPTIONS.

                  a.       INITIAL NON-STATUTORY STOCK OPTION GRANT. In
connection with the commencement of your employment, the Company will recommend
that the Board of Directors grant you a non-statutory stock option to purchase
180,000 stock shares (subject to stock splits and recapitalizations) of the
Company's Common Stock (the "Shares") with an exercise price equal to the fair
market value on the date of the grant ($5.25 per share). These options will vest
in accordance with the following schedule: twelve and one-half percent (12 1/2%)
of the Shares shall vest on the six-month anniversary of the Start Date and
1/48th of the total number of Shares shall vest each month thereafter. Vesting
will, of course, depend on your continued employment with the Company. These
options will be subject to the terms of the Company's 1997 Stock Option Plan and
the Stock Option Agreement between you and the Company.

                  b.       INITIAL INCENTIVE STOCK OPTION GRANT. In addition,
the Company will recommend that the Board of Directors grant you an incentive
stock option to purchase 10,000 stock shares (subject to stock splits and
recapitalizations) of the Company's Common Stock (the "Shares") with an exercise
price equal to the fair market value on the date of the grant ($5.25 per share).
All of the Shares subject to this Option shall be fully vested on the completion
of the Company's Initial Public Offering. The option will be an incentive stock
option to the maximum extent allowed by the tax code and will be subject to the
terms of the Company's 1997 Stock Option Plan and the Stock Option Agreement
between you and the Company. The Company will allow the exercise of the option
by delivery of a recourse promissory note in the principal amount of the
exercise price and requiring the shares acquired to be pledged to the Company to
secure payment of the note.


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                  c.       SUBSEQUENT OPTION GRANTS. Subject to the discretion
of the Company's Board of Directors, you may be eligible to receive additional
grants of stock options from time to time in the future, on such terms and
subject to such conditions as the Board of Directors shall determine as of the
date of any such grant.

         6.       BENEFITS.

                  a.                INSURANCE BENEFITS. The Company will provide
you with standard medical, dental, long-term disability and group life insurance
benefits in accordance with the Company's policies in effect from time to time.

                  b.                VACATION. You will be entitled to 15 days of
paid vacation each year for your first four years of employment. Beginning on
the fourth anniversary of your hire date, you will accrue an additional day of
vacation for each additional year of employment as provided under the Company's
vacation policy.

                  c.                401(K) PLAN. You will be eligible to
participate in the ROSETTA INPHARMATICS 401(k) Plan on the first day of the
quarter following your date of hire.

                  d.                FLEXIBLE SPENDING ACCOUNT. You will be
eligible to participate in a Flexible Spending Account on the first day of the
month following your date of hire.

         7.       CONFIDENTIAL INFORMATION AND INVENTION ASSIGNMENT AGREEMENT.
Your acceptance of this offer and commencement of employment with the Company is
contingent upon the execution, and delivery to an officer of the Company, of the
Company's Confidential Information and Invention Assignment Agreement, a copy of
which is enclosed for your review and execution (the "CONFIDENTIALITY
AGREEMENT"), prior to or on your Start Date.

         8.       CONFIDENTIALITY OF TERMS. You agree to follow the Company's
strict policy that employees must not disclose, either directly or indirectly,
any information, including any of the terms of this agreement, regarding salary,
bonuses, or stock purchase or option allocations to any person, including other
employees of the Company; provided, however, that you may discuss such terms
with members of your immediate family and any legal, tax or accounting
specialists who provide you with individual legal, tax or accounting advice.

         9.       TERM. Your employment with the Company will be on an "at will"
basis, meaning that either you or the Company may terminate your employment at
any time for any reason or no reason, without further obligation or liability.


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         We are delighted to be able to extend you this offer and look forward
to working with you. To indicate your acceptance of the Company's offer, please
sign and date this letter in the space provided below and return it to me, along
with a signed and dated copy of the Confidentiality Agreement. This letter,
together with the Confidentiality Agreement, set forth the terms of your
employment with the Company and supersede any prior representations or
agreements, whether written or oral. This letter may not be modified or amended
except by a written agreement, signed by the Company and by you.

Very truly yours,

ROSETTA INPHARMATICS, INC.

/s/ John J. King

John J. King, II
Senior Vice President, COO


ACCEPTED AND AGREED:

GREGORY SESSLER

/s/ Gregory Sessler
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Signature

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Date

Enclosure:        Confidential Information and Invention Assignment Agreement
                  Benefit Summary