Marilyn Horne is commonly thought of as a mezzo, because so many of her roles are mezzo roles. Horne knows better, and that is why she lists herself merely as a "soprano," as in the program for last night's splendorous recital at the Kennedy Center.

Horne is unique, at least for our time, and that is the point that trips the conventional categorizers. There are really two voices in Horne, meshed to perfection. One is the breathtaking coloratura with which she projects bel canto scales and ornaments of, say, Rossini, whose music took up the first half of last night's concert. It is perhaps the most breathtakingly accurate and agile voice now before the public.

The other voice is a thrillingly full contralto, a voice with spectacular breath control, capable of the most refined nuances of color.

The combination is not just a great voice, it is a phenomenon -- one the example of which will dog those singers who invite comparison for generations to come.

In last night's program -- a generous offering that went on for 2 1/2 hours -- it was the contralto that reigned supreme, starting with the sensitive shading to the slow opening of "O patria!" from Rossini's "Tancredi." This part of the voice kept getting more and more vibrant -- through a beautifully controlled "Long Time Ago" from Copland's "Old American Songs," an exquisite "Connais-tu le pays?" from Thomas' "Mignon," a seductive but controlled "Mon coeur s'ouvre a ta voix" from "Samson et Dalila" to a racy "Habanera" from "Carmen."

Her Rossini did not quite have the lung power that it sometimes does. But in the quieter moments " Horne's clarity of line and her precision of ornamentation were incomparable.

The Washington Performing Arts Society provided a respectable free-lance orchestra. Horne's usual pianist, Martin Katz, conducted crisply.