First there was Vernon and Irene Castle, then Fred Astaire and Ginger Rogers -- on screen, of course -- but since then we are the first to come on stage with this type of dancing." Pierre Dulaine, artistic director of American Ballroom Theater, is attempting, in his ever-so-charming Gallic way, to explain why he and the seven other members of his troupe have been playing to sold-out houses and critical raves since their debut in the fall of 1984.

Once one has seen Ballroom Theater work its magic, however, explanations seem beside the point. Gliding, swooping and swirling across the floor, these performers -- aided immeasurably by the choreography of John Roudis -- turn the familiar steps of the waltz, fox trot, tango, lindy and other exhibition dances into ineffable stage pictures. Each of the four couples has its specialty. Dulaine and codirector Yvonne Marceau are masters of the sleek, lift-filled adagio form; John and Cathi Nyemchek do a mean quickstep and anything else requiring fast feet and loads of spunk; Gary and Lori Pierce exude a tender lyricism; and Richard and Bonnie Diaz -- who make their Washington debut tonight when the company opens its week-long run at the Terrace Theater -- shine in the sizzling Latin-style numbers.

"I'm the grandpapa of the group," says Dulaine, 41, "and I hope that our work is giving other 40-year-old people, as well as younger people, something they can relate to. If you are older than that, you would have grown up to that music and style of dancing. But if you grew up on disco and rock, you haven't felt what we do in a physical sense. You have not lived it."

Dulaine himself is a prime example of someone who has "lived" ballroom. Of French and English parentage, he grew up in Birmingham, England, and took his first social dancing lesson at the behest of his father.

"I was 14," he recalls, "and I liked it so much that by the time I was 16 I was teaching six or seven nights a week." At 21, Dulaine became a performer on the exhibition dancing circuit, and, having fallen under the influence of the great British dancer/coach John Delroy, eventually "crossed the bridge from ballroom to adagio."

"So I began looking for a partner who had come from the ballet world. I went to New York City, got a job teaching at the Arthur Murray Dance Studio, and it was there that I met Yvonne, who was also an instructor. My dreams were answered: She was light, petite, ballet-trained." The partners went on to win numerous worldwide competitions, but it was not until veteran ballroom dancer and coach Roudis came into their lives that the seeds for Ballroom Theater were sown.

"It was suggested that we meet this 82-year-old man with 0 million lifts in his repertoire," says Dulaine, "and we've been with him ever since." In 1982, the partners, together with the Pierces, presented their first "theatrical" venture at Jacob's Pillow. It proved to be such a smash that the four dancers incorporated as American Ballroom Theater, expanded their personnel, and have been tripping the light fantastic ever since. Following their Kennedy Center appearances, they're scheduled to perform on the vast opera house stage of the Brooklyn Academy of Music, and then it's off to Europe for a tour of London, Lyon and Berlin.

As if that weren't enough, both Dulaine and Marceau operate their own studios (his in New Jersey, hers in Manhattan), and for the past two years they have been giving ballroom instruction to students at the School of American Ballet.

"When Peter Martins took over at the New York City Ballet, he thought it important for the dancers to look at ballroom and partnering," says Dulaine. "You know, he did cotillion dancing in Denmark, and Yvonne and I feel he's one of the greatest -- if not the greatest -- partners in the ballet world. But most of these ballet dancers are discovering social dancing for the first time. They are used to showing their fronts, but in ballroom, the beauty and line must come from the back. We go into the floor and get our body flight from the floor -- they must work to get the same flow.

"But the steps are so easy for them." He laughs. "They cannot believe that they can move around on the floor without hurting."