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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 supplement (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 Capital, LLC ("DAC") remains the
same except that DAC has transferred it ownership interest in the warrants and
the shares underlying the warrants to Diaz & Altschul Group, LLC ("DAG"). As
such, DAG 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 DAC. 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 JANUARY 13, 2000