EXHIBIT 5.01

                               September 25, 1998

Integrated Systems, Inc.
201 Moffett Park Drive
Sunnyvale, California  94089

Gentlemen/Ladies:

         At your request,  we have examined the  Registration  Statement on Form
S-8 (the  "Registration  Statement")  to be filed by you with the Securities and
Exchange  Commission  (the  "Commission")  on or  about  September  28,  1998 in
connection with the  registration  under the Securities Act of 1933, as amended,
of an  aggregate  of  2,000,000  shares  of your  Common  Stock  (the  "Stock"),
1,000,000  of which are subject to  issuance  by you upon the  exercise of stock
options  granted or to be granted by you under your 1988 Stock Option  Plan,  as
amended (the "1988 Plan"), and 1,000,000 of which are subject to issuance by you
upon the  exercise of stock  options  granted or to be granted by you under your
1998 Equity  Incentive  Plan (the "1998 Plan").  The 1988 Plan and the 1998 Plan
are collectively  referred to in this letter as the "Plans").  In rendering this
opinion, we have examined the following:

         (1)      your   registration   statement   on  Form  S-8  (File  Number
                  333-12799) filed with and declared effective by the Commission
                  on September  27, 1996,  together  with the  Company's  Bylaws
                  filed as a part thereof;

         (2)      your registration  statement on Form 8-A (File Number 0-18268)
                  filed with the  Commission  on January 29, 1990, as amended by
                  Amendment  No. 1 filed with the  Commission  on March 1, 1990,
                  together  with  the  order  of  effectiveness  issued  by  the
                  Commission therefor on March 5, 1990;

         (3)      the Registration  Statement,  together with the Exhibits filed
                  as a part thereof;

         (4)      the Prospectuses  prepared in connection with the Registration
                  Statement;

         (5)      the minutes of meetings and actions by written  consent of the
                  shareholders and Board of Directors that are contained in your
                  minute books that are in our possession; and

         (6)      the stock records that you have provided to us  (consisting of
                  a letter dated  September 24, 1998 from your  transfer  agent,
                  ChaseMellon Shareholder Services, stating the number of shares
                  of your capital stock issued and  outstanding on such date and
                  a  report  of the  number  of  options  that  are  outstanding
                  respecting   your  capital  stock  and  of  any  other  rights
                  outstanding  to your purchase  capital stock that was prepared
                  by you and dated September 23, 1998).

         (7)      a  Management  Certificate  addressed  to us and dated of even
                  date  herewith  executed  by the  Company  containing  certain
                  factual and other representations.

         We have also  confirmed  the  continued  effectiveness  of the Company'
registration under the Securities Exchange Act of 1934, as amended, by telephone
call to the offices of the Commission and have confirmed your eligibility to use
Form S-8.



         In our  examination of documents for purposes of this opinion,  we have
assumed,  and express no opinion as to, the  genuineness  of all  signatures  on
original  documents,  the  authenticity  of  all  documents  submitted  to us as
originals,  the  conformity  to  originals of all  documents  submitted to us as
copies,  the legal capacity of all natural persons  executing the same, the lack
of any  undisclosed  terminations,  modifications,  waivers or amendments to any
documents  reviewed by us and the due  execution  and delivery of all  documents
where due execution and delivery are prerequisites to the effectiveness thereof.

         As to matters of fact relevant to this  opinion,  we have relied solely
upon our  examination  of the  documents  referred to above and have assumed the
current  accuracy  and  completeness  of the  information  obtained  from public
officials  and  records   referred  to  above.   We  have  made  no  independent
investigation or other attempt to verify the accuracy of any of such information
or to determine the  existence or  non-existence  of any other factual  matters;
however,  we are not aware of any facts that would cause us to believe  that the
opinion expressed herein is not accurate.

         We are  admitted to  practice  law in the State of  California,  and we
express no opinion herein with respect to the  application or effect of the laws
of any jurisdiction other than the existing laws of the United States of America
and the State of California.

         Based upon the foregoing,  it is our opinion that the 2,000,000  shares
of Stock  that may be  issued  and sold by you upon the  exercise  of (a)  stock
options  granted  or to be granted  under the 1988 Plan or the 1998  Plan,  when
issued  and  sold in  accordance  with the  applicable  plan  and  stock  option
agreements to be entered into  thereunder,  and in the manner referred to in the
relevant Prospectus associated with the Registration Statement,  will be validly
issued, fully paid and nonassessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement  and  further  consent  to  all  references  to  us,  if  any,  in the
Registration  Statement,  the  Prospectus  constituting  a part  thereof and any
amendments thereto.

         This opinion  speaks only as of its date and we assume no obligation to
update this opinion  should  circumstances  change  after the date hereof.  This
opinion is  intended  solely for the your use as an exhibit to the  Registration
Statement for the purpose of the above sale of the Stock and is not to be relied
upon for any other purpose.


                                            Very truly yours,



                                            FENWICK & WEST LLP



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