In "La Sylphide," the hero believes he has ideal beauty within his grasp only to find himself sadly deluded.

The situation was mirrored in American Ballet Theatre's performances of the ballet Saturday afternoon and evening at the Kennedy Center Opera House. There seemed every reason to suppose that both ABT casts would triumph over the difficulties posed by the work. And they did, insofar as most of the external challenges were concerned -- setting the mood, creating vivid characters, executing the steps, conveying the story clearly, and even, to an extent, mimicking the period look of early romanticism.

The artistic heart of the matter, however -- making James' pursuit of the Sylph feel like a matter of metaphysical and emotional urgency -- proved elusive.

At the matinee, Cheryl Yeager was a perfect Sylph in all respects but one. She danced exquisitely, flitting from phrase to phrase with elfin delicacy and charm. There was, however, not the slightest touch of mystery or otherworldliness to her portrayal -- nothing, in other words, to suggest why James would be so powerfully impelled to forsake the attractions of his earthly fiance'e Effie (persuasively interpreted by Amy Rose) in favor of the Sylph.

Kevin McKenzie, despite some technically ragged passages, made a convincingly impassioned James. Victor Barbee was as histrionically deft as ever as the vengeful witch Madge, and John Gardner was exceptionally sympathetic as Gurn, James' rival for Effie's hand.

Johan Renvall, in the evening performance, was both dashing in appearance and virtuosically brilliant as James. On the other hand, he seemed not at all obsessed with the Sylph (Marianna Tcherkassky) -- mildly attracted to and amused by her, perhaps, but no more than that. Tcherkassky's performance, moreover, was oddly detached -- correct, in a by-the-numbers fashion, but uninvolved. Gil Boggs' indifferent Gurn was no help, nor was Raymond Serrano's excessively monstrous witch; one ought to be able to feel a few residual pangs of empathy for Madge to balance the dramatic equation.

At both performances, the two-act "Sylphide" was prefaced by another ballet, a programming tactic that makes little sense. Presumably ABT does this to make customers feel they're getting value for their money, and to utilize its dancers to the maximum. But the effect is to dilute the dramatic impact of "Sylphide," and to force upon the added ballet the effect of an aperitif.

Be this as it may, Alessandra Ferri was more effective in her debut in the matinee's "Stravinsky Violin Concerto" than one might have anticipated from this theatrically inclined dancer, though she had problems with the contortionist, all-fours turns of the first duet. Ethan Brown, also making a debut in this Balanchine opus, was reasonably sturdy for a first time out. The dancer who thus far comes conspicuously closest to mastery of the Balanchine-Stravinsky idiom is Robert Hill, who's especially impressive in the last section of the "Toccata" movement, with its repeatedly stabbing diagonals.

In the evening, Wes Chapman, appearing for the first time in the cast of Paul Taylor's "Sunset," plunged effectively into the weighted swing of Taylor's idiom, helping to bring this performance closer to the poignant potential of the work than the one seen during the opening night of the ABT run last Tuesday.