George A. Marsh, a young first violinist with the National Symphony, won a first place in the J.S. Bach International Competition here yesterday.

He was the fifth of six finalists in the contest -- three violinists and three cellists, both playing two magnificent unaccompanied works for each instrument.

First prize among the three cellists went to Steven Thomas, a young player who is the first cellist of the New Haven Symphony.

The incredibly painstaking way this competition is set up on the stage of George Washington University's Lisner Auditorium leaves the performers and the judges utterly isolated. Not only is there a screen in between but the players even have to enter on a carpet, so that the judges have no notion of the player's sex -- not that it would necessarily matter.

The audience, however, can see both the judges and the contestants; in the formality of its noncommunicabilty, one is reminded of a formulation of Beckett. One began to get the notion that Marsh would win, though, when in the final allegro of the unaccompanied C-major sonata, the most famous of the jurists, violinist Alexander Schneider -- who is a preeminent Bach performer -- started nodding his head in tempo with the music. Marsh was playing with real zest, and Schneider looked like he was just unable to maintain his judicial composure under the circumstances.

Marsh was the highest scorer on the point scale used by the three judges, with a 99 percent. And, curiously, he triumphed in the epic work after a rather tentative start. His vibrato was a little shaky in the opening adagio, but as the music became more difficult he became more in command. The second of the sonata's four movements is a stunning, and extended, fugue. From a purely digital point of view, Marsh did not meet the difficulties of its notes all that much better than the violinist who preceded him, Surgil Lee. But he was considerably more sensitive to its shifting moods and its conceptual ardors.

Lee's playing was splendid, but it did not quite have that intensity that got Schneider so involved in Marsh's finale.

Swedish violinist Semmy Stahlhammer finished third, despite a highly inflected performance. In the first movement it seemed too jagged. Also, there was a certain dullness to his tone throughout -- perhaps the fault of his instrument. But by the time he got to the finale, Stahlhammer was playing with real force -- and, based on the applause, he appeared to be the audience's favorite.

The second- and third-place winners among the cellists -- Stephen Balderston and Mark Tanner -- were less fluent.

Along with Schneider the judges were cellist Siegfreid Palm and musicologist Karl Geringer.

The contest, under the auspices of sponsor Raissa Tselentis, has been going on for 27 years. The two first winners are awarded, including all benefits, prizes that add up to an estimated $4,000 each.