"Bons temps rouler!" Day or night, that was the rallying cry at Wolf Trap's "Louisiana Swamp Romp" Sunday. The joyful refrain was invoked in one fashion or another by an impressive cast of Cajun, zydeco and blues acts, including some rarely seen on the East Coast.

The afternoon show got off to a less than promising start with a now-rollicking, now-chatty performance by blues, soul and boogie pianist Katie Webster, and the announcement that C.J. Chenier's Red Hot Louisiana Zydeco Band would not appear as scheduled. Judging by the almost inaudible response, most of the crowd wasn't aware of Chenier's exceptional group, and besides, there was some consolation to be found in extended performances by the Black Top All-Stars and, in particular, Beausoleil.

The All-Stars featured vibrant sets by three veteran blues artists seldom seen in the area: James "Thunderbird" Davis, who commanded an imposing baritone voice on ballads and jump tunes; saxophonist Grady Gaines, one of the last great exponents of raspy, honking, Texas-style tenor sax; and the sly and soulful New Orleans songwriter and guitar legend Earl King. Providing tasteful support and performing a solid and often subtly shaded set of their own were guitarist Ronnie Earl and the Broadcasters.

For sheer rhythmic thrust, melodic grace and traditional dance hall appeal, however, nothing came close to rivaling the afternoon finale by Beausoleil. With its mixture of lovely waltzes, spry two-steps, early jazz-influenced blues, original pieces and exhilarating arrangements of tunes written by everyone from Clarence Garlow to Buddy Holly, the fiddle- and accordion-powered band instantly set into motion a lawn full of dancers, along with a sizable contingent of like-minded folks near the stage, where dancing was prohibited (which is basically the equivalent of outlawing sex at an orgy).

By contrast, the highlight of the evening concert came early on with a relaxed and delightfully anecdotal performance by New Orleans singer, songwriter and pianist Allen Toussaint. The author of countless R&B hits, Toussaint shared with the rapt audience how he came to write some of his best-known songs -- for instance, how "Mother-in-Law" begat "Don't Leave Me No More" -- and frequently demonstrated his oft-overlooked skills as a pianist on everything from Chopin and Gershwin to Professor Longhair. Moreover, Toussaint's easy delivery and mellifluous voice often improved on the hit versions of "Southern Night," "Brickyard Blues" and other tunes.

The evening concluded with exuberant if hardly memorable performances by a pair of accordion-driven bands: Wayne Toups and Zydecajun, and Buckwheat Zydeco. The charismatic Toups has added an organ to a rock-oriented fusion of Cajun and zydeco styles, while Buckwheat Dural's eight-member ensemble spent much of its time onstage rambunctiously covering tunes composed by Hank Williams, Fats Domino and Bob Dylan.