Ella Fitzgerald is 67 years old, and her voice is no longer the light, sweet soprano with which she sang "A-Tisket, A-Tasket" when she was 20. But the clarity of tone and diction is still there; she knows how to treat words as poetry, not mere sound; and her technique -- timing and phrasing -- is still outstanding. So is her wit -- for example, her graphic illustration of the phrase "The way you sing off-key" in "They Can't Take That Away From Me."

It takes a bit longer than it used to for her to reach the amazing levels of agility in free improvisation that have dazzled audiences for nearly half a century. But she still gets there, given the time to warm up, and when she does it is still dazzling.

Fitzgerald reaches her top form right at the end of tonight's "On Stage at Wolf Trap" (10 p.m., Channel 32 and Maryland Public TV; 10:30 p.m., Channel 26). The program ends with the spotlight on Fitzgerald in a brief but freewheeling jam session with pianist Oscar Peterson, guitarist Joe Pass and bassist Keter Betts -- all masters of solo and collective improvisation. It is a moment of splendor in an evening marked above all by highly skilled professionalism.

Peterson, a mere youth of 60, has been in the international spotlight almost as long as Fitzgerald. His technique shows no signs of age, and his instrument (unlike hers) is easy to repair or replace. He is currently playing a Bo sendorfer, the aristocrat of pianos, and it matches exquisitely his highly polished, deeply expressive style, which blends classical solidity with the freedom and imagination of jazz. He is excellent throughout his solo segment -- at his best, perhaps, in his treatment of such works as "Take the A Train" and "Caravan" by Duke Ellington, a jazz composer whose works have become classic.

Among the fine sidemen, there are excellent solos by Pass (who proves that the amplified guitar can be used as a musical instrument) and Betts, who is also shown working in a session with preschool children as part of the Wolf Trap Institute for Early Learning in the Arts. Betts' statement that he enjoys working with children because he finds in it "the satisfaction of giving something back" is as refreshing as the music (one hour excerpted from two hours of performance) that fills the rest of the program.