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Prospectus Supplement                           Filed Pursuant to Rule 424(b)(3)
to Prospectus Dated April 7, 1997                             File No. 333-20321


                                 CEPHALON, INC.


         This prospectus supplement supplements information contained in that
certain prospectus of Cephalon, Inc. dated April 7, 1997, as previously amended
and supplemented (the "Prospectus") covering the resale from time to time of up
to 1,524,000 shares of common stock, $.01 par value (the "Common Stock"), of
Cephalon, Inc. (the "Company") by certain selling stockholders of the Company.
This prospectus supplement is not complete without, and may not be delivered or
utilized except in connection with the prospectus, including any amendments or
supplements thereto.

         The information set forth in the Prospectus under the caption "Selling
Stockholders" with respect to Diaz & Altschul Group, LLC ("DAG") remains the
same except that (i) DAG has transferred it ownership interest in the warrants
and the shares underlying the warrants to OTATO Limited Partnership ("OTATO")
and (ii) footnote (3) is deleted in its entirety. As such, OTATO shall now be
considered a Selling Stockholder for purposes of the Prospectus with respect to
the 55,314 shares of Common Stock previously beneficially owned by DAG. The
information set forth in this paragraph amends the Prospectus and is deemed to
be included in the Prospectus.



                 THE DATE OF THIS PROSPECTUS IS FEBRUARY 16, 2000