With an army of fans mustered via the Internet and word-of-mouth, Boston popsters Guster [ tickets ] used to be an example of how to make it without a major-label contract, but now they have a major-label contract. So what if there isn't much ideological purity in the world anymore? Purity of a sort can still be found in their sweet live show, as evidenced on April Fool's Day at a sold-out House of Blues in Chicago.

The trio continued to cultivate their mostly under-21 fan base with a friendly, jokey approach--they may even smile more on stage than the boy-bands that threaten to steal their constituency--starting with taking the stage to Black Sabbath's ''Iron Man.'' Four songs into the show, singer Ryan Miller ceased his attractively crystalline falsetto to invite a guy wearing an ''I Ditched Prom to Go to a Guster Show'' T-shirt onstage to slow-dance with a girl. Miller and guitarist Adam Gardner tried to lip-sync a song, just for fun, and then owned up to it halfway through. Their solid and shimmering cover of ''California Dreaming'' displayed the kind of double-edged wit that makes them interesting, while their cover of Sixpence None the Richer's ''Kiss Me,'' from TV show ''Dawson's Creek'' was less self-aware and ironic.

Audience members bopped their heads from side-to-side during new musical experiments like the band's ''Garbage Can Medley,'' in which they sang the best snippets of their self-admittedly worst songs. That so many fans knew the medley, which the band had announced on its website and debuted two nights before, proved that serious fans and serious lyrics can blend in a non-serious environment. There was hardly a song played that evening to which much of the audience could not and did not sing along.

That is, until the final encore, in which Miller asked for quiet to play and sing an unplugged version of debut-album gem ''Mona Lisa.'' The impressive acoustics of the venue and the politeness of the silent crowd made the experience like all that is Guster: very nice.