The extraordinarily provocative dance troupe that calls itself Pilobolus is back in town, this time for a week's engagement - in two distinct programs - at the National Theater.

This is one of those events where the intermissions are almost as divert as the performances, simply because the esthetic riddles Pilobolus poses are riddles Pilobolus poses are such a prolific source of controversy.

Whatever else one might conclude about Pilobolus, it is clearly a startling and fascinating evening of theater they offer.

It should also be said at the outset that the four men and two women who constitute the ensemble are a superbly polished group of performers. The work that they do, however, much of it collectively choreographed by this directorless troupe, is so different than most other dance that it is exceedingly difficult to characterize in words.

The word Pilobolus is the name of a fungus, and it's an intimation of the group's fascination with organic shapes ands convolutions. By a technique they call "body linkage," they use their bodies to emulate such shapes, intertwining limbs and heads and torsos along unexpected circuits to arrive at their visual goals.

Moses Pendleton, one of the cofounders of Pilobolus, once likened the group to a "collective muscle." The interlocked patterns the bodies generate demand exceptional strength, flexibility, balance, coordination and, above all, ingenuity.

As the troupe evolved from its beginnings six years ago, it has added increasingly theatrical dimensions and suggestive symbolism to its earlier, more abstract work. "Ocellus," for the four men, was the oldest (1972) piece on last night's program, and it shows the original emphasis on biomorphic shape in the purest state. Later works, such as "Monkshood's Farewell," the brand new "The Garden Gate" - a solo by and for Martha Clarke - and "Untitled," exhibit two other principal concerns: a kind of insidious eroticism, and an obsession with medievalism.

The images which sustain these works are invariably imaginative, sometimes quite funny, and often disquietingly bizarre. In "Mondshood's Farewell," for example, the men contort their bodies into gargoylish lumps and foreshortenings. This kind of grotesquerie is an echo of Paul Taylor's eccentric works, like "Churchyard" and "Book of Beasts," also with medieval flavorings. In "Untitled," the women lift voluminous Victorian skirts to "give birth" to naked men.

Perhaps strangest of all is the use of time. The movement seems to have duration, but no rhythm. And the works evolve formally not by kinetic development, but by a sort of vegetative process - one shape sprouts from another.

Pilobolus stands at the conjunction of dance, mime, gymnastics, charade and side-show, and half the fun is puzzling out just what it is they're up to.