Galt MacDermot, composer of the rock music for the Broadway hit "Hair", is collaborating with Joseph Papp, founder and producer of the New York Shakespeare Festival, on a rock opera derived from Shakespeare's "Troilus and Cressida."

Mr. Papp, author of the libretto, will present "Cressida" next season for eight weeks at the nonprofit Public Theater, the festivals downtown base.

He first produced "Hair" there two years ago, before it was acquired for Broadway by Michael Butler.  Current productions of the show - in New York, London, Munich and Stockholm - are bringing Mr. Papp about $1000 a week in royalties (three-quarters of one percent of the producer's share).  Mr. Papp, who will also receive 10 per cent of Mr. Butler's share of the movie rights, has earmarked the money for the festival's operating expenses.

In addition, he has made an arrangement whereby Stuart Ostrow, producer of the Tony award winning "1776" will have first call on transferring productions from the Public Theater to Broadway.

The running time of "Cressida" will be about 2 1/2 hours, and hour less than the Shakespeare.

Only with rock music, Mr. Papp said, "do Shakespeare's words stand out.  Operatic singing usually obliterates the dialogue, but rock, on the other hand, enhances the language and gives it a new dimension."

Mr. Papp said he was not certain whether he would direct "Cressida."

Copyright The New York Times Company.  All rights reserved.