For the past 20 years Dick Cerri, the audio encyclopedia of folk music in Washington, has had a quarter bet with singer John Denver.

"If you got two people together" in the '60s, "John would get up and sing," Cerri remembers. "He would do this for nothing. I said on the air one night that this was unusual and I felt he was so talented one day he would be a real big star . . . and forget the little people he met along the way, including myself. I bet him 25 cents he would do that.

"The last time I saw him, there were so many people around him, I was way in the back of the room. I heard 'Dick! Dick!' And it was John. And I said I would not ask him for the quarter today."

As Cerri sits in a tiny office on MacArthur Boulevard, he tells that story nonchalantly, like he tells the ones about other folk and bluegrass meteors: Ian and Sylvia, Bob Dylan, Tom Paxton, Judy Collins, Joan Baez, Ricky Skaggs.

It's apparent that many of them, besides Denver, haven't forgotten Cerri. A notable group marked Cerri's 25 years on Washington radio Saturday night at Lisner Auditorium. Schooner Fare, Christine Lavin, Kate Wolf, Tom Rush, Jonathan Edwards, Cathy Fink, the Seldom Scene and Dave Guard, one of the original members of the Kingston Trio, performed in a benefit for the World Folk Music Association. Cerri is president of the group, incorporated in 1982, and his home is what they called a crash pad in the sw,-1 sk,1 folk heydays; Paxton, Wolf and Carolyn Hester were Cerri house guests last week.

Tall, heavyset and relaxed in a blue cardigan, Cerri, 49, has a bell-like laugh and a deep voice that has the rush and push of a salesman's. But he gets edgy when asked about his reputation as folk music's local guru.

"I don't have a degree in musicology or years of study on a musical instrument," he says. "To me those are the experts. I'm a little embarrassed by that label. I have never looked at this music as scholarship. I guess my mind remembers a lot of things about the people I met and the music I played. I consider myself a professional spectator."

What Cerri has been watching and promoting has been the special quality of folk music. "Folk music reflects what is going on at the moment and that is always changing," says Cerri. "What was happening in the 1960s made it a natural period for folk music to come up and be as popular as it was. Now we don't have 'pop' folk music on the charts. But what is happening in folk music today is much more exciting than in the 1960s. People in the field are more talented, and they're in it because they want to be, not because it is a fad."

Each Sunday night from 8 to 11:30 Cerri holds forth on WLTT-FM (94.7). Like other specialty deejays, he brings more love and history than raw preparation to his show. He will play several artists' versions of a single song, salute birthdays and anniversaries and intersperse some of his thousands of hours of personal interviews. He even tracks down the once-famous for his listeners. Last week they learned that Chad Mitchell is now entertainment director on the Delta Queen.

"One of the reasons I might get a lot of listeners is that Sunday night in radio is considered a graveyard shift," he says, laughing. From his bag of 90 letters a week, Cerri knows a lot of lawyers are listening. The most recent ratings survey shows his is the second-most-listened-to Sunday night show, following WHUR-FM (96.3). More men between 18 and 49 are listening to Cerri than any other show at that time. His popularity with males surprises him but he has an idea: "With so many men listening, more women should go to folk events."

Twenty-five years ago, when Cerri moved from his home town of Utica, N.Y., he and folk music hit Washington at the same time. In the late 1950s the only regular place to hear folk in Washington was the Showboat, which had an open mike "hootenanny" on Sunday nights, and "if you got 50 people, you were lucky."

The Shadows, precursor of the now-defunct Cellar Door, which opened in 1961 as a beer hall for Georgetown students, and all the new artists -- from Collins to Denver to Emmy Lou Harris (then still in high school) -- found their way to the club. The same year Cerri started his show, six hours of folk, seven nights a week on WAVA-FM (105.1). His current radio show is going into syndication this spring, and he does a showcase at the Birchmere the third Tuesday of every month.

Back then, Cerri recounts, "I didn't have a label for it. I liked the Kingston Trio, the Brothers Four, the Limelighters, because they were great. Then all of a sudden someone said, 'That's folk singing.' "

He asked a music distributor for some folk music, and he said, " 'We have a couple of records selling well by a singer named Joan Baez.' I said 'Joan who?' That night I played Joan Baez and the phone started to ring all night long."

For two years in the early 1970s Cerri abandoned his announcer's perch for the management and recording business. There's a framed copy of his "Fox on the Run" record label on the wall. "Artistically it was a success," he said, "but I lost my tail."

Even though that might have been a low point, it still had the constant that has kept Cerri fresh and interested -- "meeting the people who were making the music, being able to play their music.