Let's make no bones about it: soprano Phyllis Bryn-Julson is one of the half-dozen greatest singers of our time. She is in a class with such artists as Gedda, Fischer-Dieskau and Schreier, and miles above interesting acrobats like Sutherland and manufactured personalities like Pavarotti.

She has proved it time and again here in Washington, which is fortunate to be her hometown. Usually, she proves it in rather small concert halls that are not filled, because she doesn't sing the soprano top 40; she sings contemporary music and neglected music of the past. She proved it again Saturday afternoon in the tiny Terrace Theater (which was not filled) with a program that was a constant revelation and delight.

She sang beautifully in English, French, German and Finnish, not to mention a "Vocalise" by Charles Boone that went beyond mere language into pure sound for some of its effects. The program included a few relatively familiar pieces by Debussy, Sibelius and Ives, but most of it was dedicated to such composers as David Del Tredici (who has begun to attract widespread attention), Alban Berg (who should be better-known than he is) and Jean Papineau-Couture, one of Canada's leading composers.

The music was selected as exquisitely as it was sung. Unfamiliar highlights included seven "Quatrains" by Papineau-Couture: succinct, epigrammatic pieces, haiku-like in effect, each creating a striking mood and image in a very brief span. There were also six songs by Del Tredici to texts of James Joyce. Although these include the first music he ever composed, two of the songs, "Bahnhofstrasse" and "Alone," received their world premiere performances Saturday. The vocal writing is rather simple (though sometimes quite angular) and has the direct emotional expressiveness that is winning the composer a growing audience. The piano part is considerably more elaborate; originally the composer envisioned himself as the pianist. But on this occasion (although Del Tredici was present), the piano was played by Bryn-Julson's husband, Donald S. Sutherland, a pianist fully worthy of the singer.

Del Tredici did take the keyboard for Bryn-Julson's final encore, another of his songs, but the most striking encore was a song by Phillip Rhodes about a mythical monster that eats rocks and makes obscene phone calls.