WHAT COMES FROM Philadelphia, has 64 legs, and moves like a sylph? The Pennsylvania Ballet, which will be making its Kennedy Center debut Tuesday night, beginning a week of performances at the Opera House. The troupe was booked within the past few weeks to partially fill the void left by the cancellation of the American Ballet Theatre season scheduled to start Dec. 4.

Though the Pennsylvanians have appeared previously in Washington -- most recently at the Carter Barron Amphitheatre in the summer of 1977 -- this is the first time the company will be seen in a prime downtown setting. It's high time too, for this is not just "another" ballet outfit but a major national troupe.

By virtue of the expertise and polish of its dancers, the depth and variety of its repertoire, the duration of its season and the degree of its national exposure, the Pennsylvania Ballet falls next in rank after the New York City Ballet, American Ballet Theatre and the Joffrey Ballet among the country's sizable, classically oriented dance companies.

Founded by the indefatigable Barbara Weisberger (still the troupe's overall director) in 1963 with the help of a seed grant of several million dollars from the Ford Foundation, the company in its earlier years relied heavily upon the counsel and assistance of George Balanchine, a number of whose ballets became the mainstay of the Pennsylvania repertoire. Since then, the choreographic assets have much expanded in number and direction, keeping pace with a brisk growth in subscription sales and general pace of company activity.

By now, the troupe presents two seasons annually in New York -- it has been the "resident" ballet of the Brooklyn Academy of Music since 1974 -- and undertakes regular national tours to such centers as Los Angeles, Chicago, Seattle, Cleveland and Atlanta, in addition to the dozen or more solid weeks of annual performance on its home grounds, at the Academy of Music, the Schubert Theatre and elsewhere in Philadelphia.

In 1976, a segment of public television's prestigious "Dance in America" series was devoted to the Pennsylvania Ballet -- the only troupe beyong New York besides the San Francisco Ballet so honored thus far. The company also enjoys the exceedingly rare luxury of performing with its own musical ensemble -- the Pennsylvania Orchestra, directed by Maurice Kaplow -- both at home and on tour (Kaplow, however, will lead Kennedy Center musicians here).

The Kennedy Center programs reflect the idiomatic diversity that is one of the company's hallmarks: Balanchine's "Raymonda Variations" and "Allegro Brillante"; a popular old Ballet Russe comic romance, David Lichine's "Graduation Ball"; "Adagio Hammerklavier" by the noted Dutch choreographer Hans Van Manen; the first local showing of "Casella, 1, 3, 4" by Washington's own Choo San Goh, resident choreographer and assistant director of the Washington Ballet; Charles Czarny's sporty "Concerto Gross"; and the recently created "Aura" by Dane LaFontsee, one of the troupe's three ballet masters and once a dancer with Washington's National Ballet.

"Our repertory generally divides itself into three categories," says Benjamin Harkarvy, the troupe's artistic director for the past eight years and one of its contributing choreographers.

"There are the classics -- traditional and contemporary -- which are still the base of what we do, ballets like 'Les Sylphides' and Balanchine's 'Symphony in. C.' Then there are modern works of merit which have been performed elsewhere but which are of a suitable size and substance for our company -- pieces like Van Manen's 'Adagio Hammerklavier,' for example, or John Butler's 'Carmina Burana.' Finally, there are works which have been made on and for our company, like Dana LaFontsee's 'Aura,' for instance. It's this third category that interest me particularly -- there's always a special quality to having a ballet performed by the dancers it was made for. There's an inherent risk, of course, in any untried material, but that to me is what ballet is all about -- the constant generation of new work by new talent -- so it's a risk that must always be borne."

Harkarvy makes no effort to conceal his pleasure over the company's Kennedy Center appearance. "We've wanted this to happen for a long time.We're sorry to hav to come under such less than ideal circumstances, in the wake of American Ballet Theatre's troubles, but we're excited about the opportunity all the same."

Trying to define the unique character of the Pennsylvania Ballet isn't easy, Harkarvy says. "We're definitely a repertory company," he says. "We'are not devoted to the work of one or two dominant choreographers, and we strive as hard as we can to be respectful of the specific intentions of any work we do. I suppose this means we're not very different in concept from any large, touring repertory company like ABT or Joffrey

"But any company worth its salt has its own face, its own unique quality of projection. We feel there is a Pennsylvania look, an emanation of the dancers, of the quality of their movement. In our training we place great emphasis on the upper body and the arms, and that may have something to do with it. But ultimately, there's just a mysterious something."

Harvarky himself isn't saying so, but the Pennsylvania persona has no doubt been shaped by his own tastes and proclivities, the result of an intriguingly multifaceted career. The son of a dentist, he grew up on Long Island. As a young teenager, he was taken to see Alexandra Danilova in "Swan Lake" and was thunderstruck. "I had to go back the next evening, and the next and the next, and I drove my parents crazy." Friends introduced him into the ballet classroom, and by 17 he was dancing professionally. Substituting for his own instructor -- Mme. Elizabeth Anderson -Ivantzova -- for a time, he found himself in tremendous demand as a teacher. Realizing that his native gifts lay more in that direction, he abandoned dancing and from then on has taught, choreographed and directed a succession of companies ranging from the Royal Winnipeg Ballet to the Harkness Ballet and the National Ballet of Holland.

In 1959, he became the founder and for 10 years thereafter the director of the Netherlands Dance Theatre, which evolved into the most adventurous modern-based troupe in Europe. The company was the rage of New York in its American visit earlier this year. He returned to the U.S. in 1972, and shortly afterwards assumed his present position at the invitation of Barbara Weisberger.

"From the moment, I arrived, she gave me a free artistic hand, and we've had a very felicitious relationship ever since," he says. The results will be on view all this week from Tuesday evening through Sunday matinee.