The pop-crossover move is often scorned, and rightly so, for it usually dilutes the source material. But if you can take an underground or regional music and give it instant-gratification melodies, all objections must fade, for you've strengthened the sound, not weakened it. That's just what Smash Mouth has done with its pop makeover of the California ska-punk genre.

The Bay Area quartet scored a No. 1 modern rock hit with "Walkin' on the Sun" in 1997. Refuting the accusation that they were one-hit wonders, the four musicians are back this year with an even bigger hit single, "All Star," and an album full of potential hits, "Astro Lounge." At the bottom of the band's sound is the same syncopated beat that identifies hundreds of ska-revival acts across the land, but on top are tunes so catchy they wriggle into your brain and refuse to leave.

The secret of this success is a division of labor that has guitarist Greg Camp writing those infectious melodies (and indifferent lyrics) but leaves the singing to Steve Harwell, whose gruff but tuneful baritone gives the hooks a rock 'n' roll edge. "All Star" links its rousing chorus and its bubbly beat so tightly that it dares you not to sing along. Just as contagious are "Who's There," a song with the same story line and pop optimism as "E.T.," and "Radio," a briskly paced, brightly flavored look at the music biz.

Appearing Friday at the Merriweather Post Pavilion with Lenny Kravitz and Buckcherry.

To hear a free Sound Bite from Smash Mouth, call Post-Haste at 202/334-9000 and press 8109. (Prince William residents, call 690-4110.)