IF the United States, like Japan, designated individuals at the top of their field ''National Treasures,'' American choreographer Twyla Tharp would be one. Her work exists on two planes, often simultaneously: that of jazzy, rollicking motion and polished, poised ballet. And the sheer ecstasy she inspires in dancers brings converts into the chapel of dance.
Nowhere is this more evident than in ''Waterbaby Bagatelles,'' currently being performed as part of the Boston Ballet's American Dance Festival II -- the second in a two-part showcase of home-grown choreographer talent. This dance, which premiered here last year, is full of Tharpisms that make her work quintessentially American: funky solos, show-off tactics, cocky strutting, and sassy humor. But her dance vocabulary also includes precision, an unerring sense of direction, and smoothly coiled energy.
A bagatelle is defined in three ways -- any of which could point to Tharp's idea for the title: 1) something of little importance or value; a trifle; 2) a game something like billiards; and 3) a short musical composition.
The first impression of ''Waterbaby Bagatelles'' is visual: The dance opens with a bare stage except for four rows of Mediterranean-blue fluorescent bars, which run horizontally above the dancers' heads. The Santo Loquasto costumes bring out the theme of playing in water -- in fact, the first group of female dancers in silvery-blue looks like escapees from an Esther Williams movie. The music in Part I (much of it played by the Kronos Quartet) is monotonous percussion, but as the dance evolves, the sounds become more varied as orchestration is added. The momentum builds until it breaks out into a frenzied, joyful free-for-all.
''Waterbaby Bagatelles'' showcases the Boston Ballet's men: They strut and preen while female dancers, playing groupies, watch and react to their every move. The women bob and wriggle as if caught in some fantastic current or carried along on successive waves.
Tharp has a smart, sly edge to her choreography that gives it a hip attitude. The dancers respond to her direction by throwing themselves into the movement, and the audience is carried along by their enthusiasm. The piece is riveting.
The other prize of the evening is ''Contra Pose,'' choreographed by Eliot Feld to the music of C.P.E. Bach. This piece is about ensemble composition and tonal studies, as if Feld were sculpting bodies into long lines and dark shapes against the cream-colored background.
While the dancers approached the Tharp piece with beautifully controlled abandon, they did not always get the hang of ''Contra Pose.'' A poor choice involved the dancers ''warming up'' onstage as the audience settled in. Unfortunately, company members looked ill at ease and their movement seemed contrived.
Those drawbacks aside, the Feld is a striking exercise in shapemaking, gaining in precision and elegance what it loses in relationships among the dancers. The movement is strong, angular, and unusual, such as one instance in which the dancers look like insects on their backs, feet beating in the air.
Less successful by comparison is the third dance of the program, George Balanchine's ''Who Cares,'' set to the familiar hummable songs of Gershwin. The music, played by a live orchestra, is an especially perky arrangement of these standard tunes, and takes the raw edge off Gershwin entirely.
Balanchine invests this piece with romance and nostalgia to burn, and while it will appeal to many audiences, others will find it as sweet and sticky as a bonbon on a July afternoon.
''Who Cares'' demands a lighter-than-air touch, and the Boston Ballet plays up its tender, sentimental aspect to the hilt.
r American Dance Festival II continues at the Wang Center for the Performing Arts, Boston, through April 9.
Tharp's work is quintessentially American: funky solos, sassy humor, unerring sense of direction, smoothly coiled energy.