Five days into April 1998, on Palm Sunday, Cats's Old Deuteronomy, Ken Prymus, will make his 2,672nd -- and last -- trip to The Heaviside Layer. "Seven years is a long time," he figures. "April 21, 1991, was when I started Cats, but I wanted to leave on a nice religious day."

Marlene Danielle, a member of the ensemble, has been with the show longer--since its beginning October 7, 1982 -- but Prymus is not only its longest-running featured performer, he is also the longest-running continuous principal-player ever to work the Winter Garden -- outpointing Al Jolson, Angela Lansbury and even Cats's own Laurie Beechman.

Prymus came to Old Deuteronomy the same way he came to his four-year Ain't Misbehavin' stint: He came after another Ken -- Page -- who originated both parts on Broadway. "I am a large black man -- he is, too -- and those are the roles."

In his younger and slimmer days -- from 16 to 21 -- Prymus was part of that fresh-faced pack of musical patriots called Young Americans ("There were only two blacks out of 36 kids"). A documentary about them, bearing their name, won the Oscar for Best Documentary Feature of 1968 -- then had to return it because it had had film bookings in 1967. Prymus's other niche in screen history: in the M*A*S*H flick, he sang that playfully plaintive dirge, "Suicide Is Painless."

When he finishes his record run, he won't be going to Disney World. "I did that last summer. What I'm doing now is getting my audition chops together." Apparently, Prymus has plenty of lives -- and long runs -- left in him after Cats. "Of course, if anyone wants to write me a new musical . . ."