Who knew Selena Gomez and Justin Bieber were into jam bands? The tween power couple were guests of honor at Phish's Long Beach Arena concert last night (Aug. 15).

Bieber's guitarist Dan Kanter is a longtime Phish fan, so he's been trying to convince the 'As Long as You Love Me' singer and his lady love to come out to a Phish show for a while. What got it to finally happen? Bieber turned 18 this year, and the arena is only about 45 minutes from Los Angeles.

The Hollywood Reporter said that Bieber, Gomez and their pals watched the entire first of Phish's typical two sets, then headed backstage to hang out with the band's guitarist (and Kanter's idol), Trey Anastasio, bassist Mike Gordon and even lighting director Chris Kuroda. They even hung out with Phish's family members and took photos with their tykes, which showed Biebs sporting a Philadelphia Flyers cap and Gomez donning a floppy, boho-style black hat -- the kind only she can really pull off without looking silly.

During the second set, Sel and Biebs took part in the usual Phish glowstick war. Cute! A source told the mag, "They were having a blast." It sure sounds it! Ah, to be young, rich and Phish fans. We're just glad the clean cut pair didn't join the long list of Phish fans with drug arrests.

Watch the Justin Bieber 'As Long as You Love Me' Video