The major surprise of last night's Inter-American Music Festival concert at the Library of Congress was the brief, eventful Fantasia for String Quartet by Jose' Serebrier, who is probably best known as a conductor in this country but is also one of Latin America's leading composers. Composed when Serebrier was in his early twenties, the Fantasia dates from 1960, but in its musical vocabulary and rhetoric, it could be contemporary with the quartets of Ravel and Debussy. In impact, it is a veritable "1812" Overture of string quartets--short, tuneful, dramatic in some of its key episodes and smashing in its conclusion.

Last night, it appeared in rather serious company, with the melodious but austere Quartet No. 17 of Villa-Lobos, the pensive Quartet No. 4 of Vincent Persichetti and the compact, complex Quartet No. 6 of Ezra Laderman. By any strict accounting, the youthful Serebrier opus is probably a bit obvious compared to these other works--but the audience clearly loved it, and the audience was quite right.

Part of this music's charm last night--a very large part--can be credited to the performance by the Audubon String Quartet, whose youthful exuberance (tempered by superb discipline) clearly matches the spirit of the music. But every item on the program was well-served by this ensemble: the Villa-Lobos, where more than once a single melodic line flew around the circle from one instrument to the next with perfect coordination; the Persichetti, whose curious sounds and strange turnings seemed perfectly natural, and above all, the Laderman, which is dedicated to the group and named "The Audubon" in their honor.

Laderman's quartet, which crams thematic material for several movements into one, was the most substantial work on the program, as it was earlier this year in the Kennedy Center-Friedheim Competition, where it was performed by the same group. Their interpretation, which was very impressive at that time, has acquired perceptibly more power and polish since then. The music's continuities and contrasts are presented with more clarity, and the flow and interaction of its ideas seem more natural, more inevitable than they did a few months ago.

American string quartets have been growing in number and quality at a dazzling rate in recent years. On the evidence of last night's performance, the Audubon Quartet seems ready to take its place among the best of these groups.