PETALUMA, Calif.--The shrieks, heard from a block away, were a dead giveaway that this wasn't going to be another ho-hum folk rock show in Northern California.

Sure, 22-year-old Colbie Caillat makes the kind of folksy pop that has long been a Cali staple. But rarely does that sound attract young fans in droves, propelling a singer from cafes to the top of the Billboard charts almost overnight.

It did just that Thursday night, with more than 200 fans, mostly girls between 12 and 25, packing into the Mystic Theatre to catch a glimpse of the MySpace phenom.

In an hour-long set that was both cliché-laden and modestly infectious, Caillat didn't disappoint.

Following a solid opening set from singer-songwriter Chris Pierce, every song break on the PA, slight dimming of the slights, and movement by the stage hands was greeted by raucous screams and chants for "Colbie! Colbie!"

Taking the stage with a five-piece backing band, Caillat (pronounced like "ballet") kicked off the set with "Oxygen," the first track off her smash debut album, Coco. It was the perfect intro to Caillat's sound: endearingly simple and sweet, full of tired platitudes like, "How am I supposed to tell you I feel?" but buoyant nonetheless.

That simplicity had an obvious benefit Thursday night: a rapt audience singing along to almost every song, not just "Bubbly," the ultracatchy single that launched Caillat's success.

Caillat introduced the song "Capri" by explaining that it was about a friend and was "about becoming a mother and taking care of the baby inside her," and then sang the opening line, "She's got a baby inside her." This was music no deeper than the kiddie pool, but it didn't matter--the kiddies ate it up.

Other influences trickled in as well, from the reggae-tinged "Tied Down," which included a verse of Bob Marley's "One Drop," to the blue-eyed soul of "Feelings Show."

But tucked toward the end of the set was a hint that Caillat might be more than the O.C. version of Jewel: a cover of Lauryn Hill's "Tell Him," a moving, soulful track that lent layers to an otherwise simple set. If Caillat can translate that influence into an authentic depth of emotion--and at 22 she's got plenty of emotion ahead--this might lead to more than a splash of "Bubbly"-gum folk pop.