REMEMBER doing "The Monster Mash?" The "graveyard smash" is 20 years old this year and is exhumed each Halloween to thrill and chill a new generation of groovy guys and ghouls. The moldy oldie has earned a coffin full of cash for singer and cowriter Bobby (Boris) Pickett.

"I got the nickname 'Boris' just because I was doing the Boris Karloff imitation," says Pickett, 43, who adds that he does about 60 other impersonations, with varying degrees of expertise.

Pickett planned an acting career, so at age 23 he left Somerville, Mass., for Los Angeles, where he did television commercials and joined a rock 'n' roll band, the Cordials. "It was an a cappella, R&B singing group; four funky Italian boys and me, the token WASP. Our first job was singing for spaghetti dinners at an Italian restaurant on Friday nights. On the Diamonds' hit 'Little Darlin',' I used to do the deep bass part as Boris Karloff -- it really used to crack the audiences up."

Pickett left the Cordials "when I finally got an agent, and then he died of a heart attack two weeks later. So I called Cordials leader Lenny Capizzi and said, 'Maybe we should do that novelty record you suggested.' "

"The Monster Mash" was written in three hours "one Saturday morning in June 1962," Pickett said. "We knew just what we wanted: Boris Karloff as the mad scientist, on a slab in the lab, describing all the latest monster dance crazes. I thought at first it would be the Twist, Lenny thought it should be the Mashed Potato. We wound up inventing our own dance, fortunately."

For the staggeringly low sum of $300, producer Gary Paxton (lead singer for the Hollywood Argyles: remember "Alley Oop"?) called up some musician friends, including Leon Russell, then working as a session piano player, and dashed off the beastly boogie in three hours.

Those six hours of work have netted Pickett more than $250,000 in royalties in the past 20 years, and the money keeps pouring in.

"The first time around, in '62, I think it sold 1 1/2 million copies and went No. 1 nationally," Pickett said.

The song was rereleased in 1969, then in 1973, Pickett said, a Minneapolis station played it as an "oldie but goodie, and suddenly all these kids were tying up the switchboard for it. So it took off again, sold twice as many copies, and went No. 1 internationally. And of course, they always drag it out around this time of year."

To promote the record, Pickett did the obligatory personal appearances and sock hops. "At first I had a beach-boy image," Pickett says. "But in '73, I took 10 Crypt Kickers out on the road, the whole shebang. Our bus broke down on the way to St. Louis on a dark and stormy night. We got out and there was this sign (lightning flash): 'YOU ARE IN FRANKENSTEIN, MISSOURI.' "

The hit, a Halloween party staple, was even lauded by the monstermeister himself. "Boris Karloff actually sang the song on one of Dick Clark's television specials," Pickett says. "He liked the record a lot. He tried to buy it in a record store, and everyone recognized him and went crazy."

Pickett also created an album in the same vein, crawling with cute cuts with monstrous monikers like "Skully Gully," "Me and My Mummy " and "The Transylvania Twist." Pickett made a few more stabs at silly singles, including "King Kong, Your Song" and "Star Dreck" (which became the most requested song on Dr. Demento's nationally syndicated bizarre record program), but nothing ever topped "The Monster Mash."

Pickett, who splits his time between New York's Upper West Side and Los Angeles, is still plugging away at his acting, and avoids roles requiring a Karloff copycat. "Right now, I'm mostly working Off-Broadway and in little theater and doing voice-overs for radio and TV commercials," says Pickett, who has just completed a science-fiction movie, "Strange Invaders" and a made-for-cable movie with Orson Welles.