EXHIBIT 5.1


Morgan, Lewis & Bockius LLP
1701 Market Street
Philadelphia, PA 19103

April 23, 2001

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

Re:   Cephalon, Inc. - Registration Statement on Form S-3 Relating to the
      Registration of 750,000 Shares of Common Stock, $.01 par value

Ladies and Gentlemen:

We have acted as counsel to Cephalon, Inc., a Delaware corporation (the
"Company"), in connection with the preparation of the subject Registration
Statement on Form S-3 (the "Registration Statement") to be filed with the
Securities and Exchange Commission (the "Commission") under the Securities Act
of 1933, as amended (the "Act"), relating to 750,000 shares (the "Shares") of
the Company's common stock, par value $.01 per share, to be sold by the Selling
Stockholder named in the Registration Statement.

In rendering the opinion set forth below, we have examined the Registration
Statement and the exhibits thereto, certain records of the Company's corporate
proceedings as 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 submitted to us as originals and the conformity
with the originals of all documents submitted to us as copies thereof.

Based on the foregoing, it is our opinion that the Shares have been 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. 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.

The opinion expressed herein is solely for your benefit and may be relied upon
only by you.

Very truly yours,

/s/  Morgan, Lewis & Bockius LLP