It was late April, 1978, and the Washington Bullets were winning their playoff series against San Antonio. Bullets Coach Dick Motta heard an announcer say: "The opera isn't over until the fat lady sings." Motta liked the phrase and when the Bullets lost the next game, Motta used the phrase.

The slogan caught on, and the Ron Lowenthal Agency, which was handling promotions for the Bullets, went out and found 27-year-old Betty Clark, a singer-actress from Gaithersburg, to bring the "Fat Lady" character to life. At the next Bullets home game, Clark was clad in a plunging yellow gown and a Wagnerian horned hat, carrying a sword and shield. She was designated the Bullet's official "Fat Lady."

The Bullets beat San Antonio, then knocked off Philadelphia and, inspired by the Fat Lady and the powerful play of Wes Unseld and Elvin Hayes, defeated Seattle for the National Basketball Association title, Washington's first world championship since 1942.

Fat Lady mania swept Washington that spring. When the Bullets returned to a standing-room crowd at Dulles Airport, thousands of people sported the latest fashion--the Fat-Lady T-shirt. Clark sang, but she was drowned out by applause for the team.

Now, Clark, 31, is working for the Roundhouse Theatre, where she directs a 105-member children's chorus, performs with the theater group, and does some of the administrative work. She has also just completed a film scheduled to appear on public television in 1983.

For a while, she recalled last week, "I did a couple of nightclub gigs that stemmed out of the Fat Lady stuff," but the slogan died shortly thereafter.

She says that her weight is a "shade less than what I was," and now candidly admits that "basketball was never my favorite."