The Turtle Island String Quartet plays instruments that are strictly associated with classical music—cello, viola and violin—but when their bows touch the strings, the aural melange that erupts stands tradition on its civilized ear. Mark Summer whacks at his cello like a blissed-out rock drummer. Her fingers skipping over the viola, Irene Sazer saws out a country twang. Darol Anger strums his violin with a funky beat, and lead violinist David Balakrishnan improvises jazzy solos based on everything from chants to TV sitcom themes. "All violinists," he claims, "are frustrated rock guitarists."

Hey, Ludwig, snap your fingers before you roll over. By melding country, folk, rock, the classics and jazz, the iconoclastic combo has revolutionized the venerated art of the string quartet. They have won standing ovations in classical halls and at country festivals, and their second album, Metropolis, rose to the Top 20 on jazz charts last spring. "For the first time in jazz history," critic Leonard Feather wrote in the L.A. Times, "a string ensemble has shown the ability to improvise individually and swing collectively."

Adopting a Native American name for North America, the Turtle Islanders hooked up in San Francisco three years ago after meeting at various music festivals. Sazer, 29, had played with the Baltimore Symphony, and Summer, 31, with the Winnipeg Symphony; Anger, 36, and Balakrishnan, 35, honed their skills with string jazz ensembles. Unlike other quartets, they write or arrange all their own songs, and they have even invented new techniques, such as running their bows over miked instruments to mimic the sound of brushed cymbals.

The foursome, which is performing this week at the Concord (Calif.) Jazz Festival, is still winning fresh converts to the new sound of its old instruments. Says Anger: "Even stagehands with Bon Jovi on their headsets hear us and say, 'Hey, man, you play rock and roll!' "

"Now, that," says Balakrishnan, "is market penetration."