The Kennedy Center's Handel Festival closed its sixth season Saturday night in the Concert Hall with Tatiana Troyanos as the eponymous hero of "Julius Caesar" and "Ariodante." Here was a brilliant artist in her glorious prime, sharing the gift of greatness with her public. The evening, which also included eight choral and orchestral selections by Handel under Stephen Simon's direction, was a complete triumph.

Caesar's arias looked ahead to the San Francisco production that is to star Troyanos later this month. Ariodante's music recalled the Kennedy Center's opening season, when the American mezzo-soprano sang that role opposite Beverly Sills' Ginevra. Those with long memories--or even those with private tapes of that occasion--were not disappointed, for Troyanos is singing better than ever. It was unusual to hear Caesar's music sung by a famous Cleopatra. And it was a revelation to hear it in the voice intended by the composer. After the chorus "Caesar, Caesar," Troyanos came, sang and conquered. "Kneel in tribute" was a lesson in eloquence. And "Fleet o'er flow'ry meadows," the delicate dialogue with the violin, which makes no sense with the usual baritone Caesar, was unforgettable: violinist Jody Gatwood matched the singer's brilliance phrase by phrase, and Troyanos altowed a hint of portato while descending to the chest voice that gave the great Caesar a welcome touch of vulnerability.

Ariodante's "Scherza infida" was a lesson in legato and dignity, while "Con l'ali di constanza" was a coloratura showpiece that was outdone by a surprise encore of "Dopo notte" from Act III of the same opera. It was dazzling, the kind of performance for which superlatives should be reserved. Troyanos' coloratura is never an empty display of fireworks. Like a siren, her sound is sensuous, dark and seductive. Yet everything is done within the style and within the seamless voice, the divisions of her runs are clear but never vulgar, and the music is imbued with the elegance and passion that are the heart of Handel's greatness.