The biggest band in the world just played a small LA room — for a price of only $20 a head.

The Rolling Stones did an impromptu gig at the 700-capacity Echoplex on Saturday night, springing word of it via Twitter.

Hundreds of lucky fans saw the tweet and were instructed to line up at a theater across the street from the club. Fans forked over $20 each — cash only — for the intimate, one-ticket-per-customer show.

The fans had to show a government-issued ID, have their photograph taken and wear a wristband. Their names and faces were checked again at the door later that night.

The hassle seemed worth it, as the Stones played for 90 minutes, rocking new songs and old standbys.

The dinosaur rockers’ “50 and Counting” tour rolls into Los Angeles’ Staples Center on Friday night, with tickets that go for $150 to $600.