There were more empty seats than occupied ones Saturday at Wolf Trap's bluegrass matinee, but not one of the perfromers seemed to mind.

"We're really moving up," said one of the Johnson Mountain Boys. "Last night we were playing Zip's Tavern and today it's Wolf Trap." Like the other groups on the bill, they took full advantage of the opportunity.

The Mountain Boys prefer traditional bluegrass. Bill Monroe's influence surfaced not only in their choice of songs but also in the crisp exchanges between guitar, mandolin and fiddle.

Spectrum, a four-piece band, took a more adventurous route. An interesting treatment of a Billie Holiday tune allowed the band a chace to recall the loose rhythms common to several string bands back in the '30s. But Gloria Gaynor's "I Shall Survive" did little for the group and even less for bluegrass.

The Dry Branch Fire Squard gently combined traditional and contemporary approaches on a Jesse Winchester tune before Hazel Dickens joined them to sing the highlight of the show -- a litling homesick ballard about West Virginia.

Finally, Lost and Found brought their resonant harmonies to the stage. Their excellent musicianship was leavened by novelties like "Alley Cat" and inspired, no doubt, by the fact that in bluegrass, Wolf Trap is not just anothr gig.