Exhibit 5.1

                         MORGAN, LEWIS & BOCKIUS LLP
                              1701 Market Street
                            Philadelphia, PA 19103


May 28, 2002

Cephalon, Inc.
145 Brandywine Parkway
West Chester, PA 19380

Re:      Cephalon, Inc. - Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as counsel to Cephalon, Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing of the subject
Registration Statement on Form S-3 (the "Registration Statement") with the
Securities and Exchange Commission (the "Commission") under the Securities Act
of 1933, as amended (the "Act"), relating to the registration by the Company of
781,694 shares (the "Shares") of the Company's common stock, par value $.01 per
share, issuable upon the conversion of $55,000,000 aggregate principal amount of
the Company's 3-7/8% convertible promissory notes due March 29, 2007 (the
"Notes"). The Registration Statement registers the Shares for sale by certain
holders thereof, following conversion of the Notes, as described in the
Registration Statement.

In rendering the opinion set forth below, we have examined (i) the Registration
Statement and the exhibits thereto, (ii) the Restated Certificate of
Incorporation of the Company, as amended (the "Certificate"), and (iii) the form
of the Notes. We also have examined certain records of the Company's corporate
proceedings as represented by an official of the Company to be reflected in its
minute books and such statutes, records and other documents as we have deemed
relevant. In our examination, we have assumed the genuineness of documents
examined by us, the authenticity of all documents submitted to us as originals,
the conformity with the originals of all documents submitted to us as copies and
the authenticity of the originals of such latter documents. We also have assumed
that there will be no change in applicable law from the date hereof to the date
on which the Shares have been issued upon conversion of the Notes.

Based on the foregoing, it is our opinion that the Shares have been duly
authorized by the Company and, when issued and delivered in accordance with the
terms of the Notes, will be validly issued, fully paid and nonassessable.

The opinion set forth above is limited to the General Corporation Law of the
State of Delaware, as amended. We hereby consent to the use of this opinion as
Exhibit 5.1 to the Registration Statement and further consent to the reference
to our firm under the caption "Legal Matters" in the prospectus included in the
Registration Statement. In giving such consent, we do not thereby admit that we
are acting within the category of persons whose consent is required under
Section 7 of the Act and the rules or regulations of the Commission thereunder.


Very truly yours,


/s/ Morgan, Lewis & Bockius LLP