INDIGO GIRLS, GREGORY ALAN ISAKOV, Red Butte Garden Amphitheater, July 7.

The Indigo Girls, the musical duo that's become a staple of the Red Butte Garden summer concert series, took the stage Tuesday night and showed, once again, why their performances sell out every time they come to Salt Lake City.

Having just finished their 11th album, and well into their third decade of collaboration, Amy Ray and Emily Saliers exhibited their veteran musicianship in an almost two-hour set that showcased their sweet, two-part harmonies and Southern-tinged acoustic songcraft.

Featuring new tunes from their just released independent record, "Poseidon And The Bitter Bug," the two Georgia natives fleshed out their show with a musical retrospective and touchstones that had the capacity audience on its feet and singing every word.

In a highlight of the evening, Ray performed "Romeo and Juliet" from 1992's "Rites Of Passage" record. The song was the perfect, bewitching soundtrack for the twilight of a summer evening. Though it's a cover of a Dire Straits song written by front man Mark Knopfler, the Indigo Girls' version turns the boy-girl love song on its head, and carves out a place for girls who love girls.

Backed up by Julie Howe, who split time on the accordion and keyboards, the Indigo Girls reprised tunes from 1994's "Swamp Ophelia," 1997's "Shaming The Sun," 2000's "Retrospective" and their 1989 major label debut, "Indigo Girls." A hit song from that eponymous work, "Closer To Fine," led off an encore set and brought a roar of approval from the gathering. Joining the band onstage for the tune was opener Gregory Alan Isakov.

Isakov, an up and coming folk/Americana artist from Boulder, Colo., was joined by a violinist, cellist and drummer onstage for eight beautiful songs that showed why he's being watched by the music industry. He's channeling the spirit and charisma of "Don't Look Back"-era Bob Dylan in songs that sound as though they've traveled from another, more thoughtful time.

As has been a hallmark of the politically active Indigo Girls, Tuesday's show also supported causes important to the band, including Amnesty International, HeadCount (a voter registration group) and local food banks.

E-mail: araymond@desnews.com