Randall Thompson's "Alleluia" was written for the opening of Tanglewood, but it could have been written for the acoustics of the National Gallery's East Garden Court. As it was performed there last night by the Genesco Chamber Singers, its seamless counterpoint, aided by the hall's natural reverberation, hovered in the air effortlessly.

The chorus, on a spring vacation tour from the State University of New York in Genesco, has a live, exciting sound. With 45 or so singers it is pretty large for a chamber chorus, and indeed, it puts out enough tone for a group twice the size.

Under the baton of Robert Isgro, this power gave the "Alleluia's" climaxes a feeling of compelling power, and, had the chorus' pianissimos been as good, the overall effect of the piece would have been stunning.

Their program touched on a variety of styles and periods from the scatological "Au joli jeu" of Janequin to a couple of pieces by members of the university faculty.

Perhaps the best singing came in Mendelssohn's "Die Nachtigal," performed with a splendid sense of line and balance. This is a small masterpiece that far outshines the rest of Mendelssohn's music for small chorus, and this performance did it full justice.

Of the new pieces on the program, Jack Johnson's "Ave Maria" setting was of considerable interest, and James Walker's "Child of the Pure Unclouded Brow" was not.

The Tremont String Quartet provided some well thought out accompaniments.