The Battlefield Band, the most influential act of the Scottish folk revival, turns 30 this year and has just released its 20th album, "Leaving Friday Harbor." The Glasgow group has managed to stay vital over all those years not by resorting to gimmicks, but by constantly refreshing its ranks with new members. Singer-keyboardist Alan Reid is the only original member still on hand, and 26-year-old fiddler John McCusker is second in seniority; he joined nine years ago when he was 17. Filling out the latest line-up are two alumni of the Scottish band Coelbeg, piper Mike Katz and guitarist Davy Steele.

Rather than simply copying their predecessors, these newcomers have rejuvenated the Battlefield Band with new compositions and their own distinctive ways of playing. The new album takes its title, for example, from McCusker's fetching instrumental, inspired by an island in Puget Sound and graced by "singing" solos from the fiddle and bagpipes. Steele contributed "One More Chorus," a comic song about musicians who refuse to leave the bar where they're playing.

Whether playing dance tunes such as "Jesse 'The Body' Ventura's Reel" or filling the songs with solos, McCusker and Katz both have a gift for sharply defining the melody. This ability has convinced Reid to de-emphasize his synthesizers in the arrangements, and thus the band sounds more traditional than it has in years past on such dialect songs as "The Straw Man" or "The Pleasure Will Be Mine."

Appearing Wednesday and Thursday at the Barns of Wolf Trap.

To hear a free Sound Bite from the Battlefield Band, call Post-Haste at 202/334-9000 and press 8129. (Prince William residents, call 690-4110.)