Enough world music devotees showed up at Johnny D's on a school night to make the performance by Zimbabwe's Thomas Mapfumo worthwhile for both artist and club. Tuesday night, a dedicated crowd grooved to midtempo rhythms anchored by the mbira, the southern African ''thumb piano" built into a gourd, for two solid sets that stretched into the night.

The show was a small triumph over adversity, as it had been rescheduled twice, in part due to Mapfumo's car breaking down somewhere on the cross-country drive from his Oregon home. Visa difficulties depleted his band, the Blacks Unlimited. The improvised lineup included a stand-in drummer, a second mbira player from Haiti, and local African music maven Banning Eyre, a longtime Mapfumo friend, on guitar.

Mapfumo's music is one of clean layers. Over a bottom made of percussion and a clean, uncomplicated bass line, the mbira distills subtle nuances out of hypnotic cycles. Guitar, trumpet, and keyboard embellish while Mapfumo's elegant, slightly gravelly voice delivers the message in Shona, Zimbabwe's main language.

Mapfumo is a national treasure of Zimbabwe and its best-known artist. His chimurenga music, named for the revolutionary war against white rule in the former Rhodesia, has been a staple of the country's cultural life since majority rule was achieved in 1980.

But as the regime of Zimbabwe's president, Robert Mugabe, grew more repressive and arbitrary, Mapfumo's straightforward democratic themes made him a potential subversive. As a precaution, he and his family left the country in 2000.

Exile carries a material cost. A stadium-filler back home, Mapfumo now ekes out a living on the US club circuit.

Mapfumo's latest releases are available only online through Calabash Music, a Boston-area company that offers world music artists better terms than do traditional labels. And financial and immigration issues tend to make the Blacks Unlimited, as Eyre joked with the crowd, ''more unlimited than usual."

A small, lean figure in a simple white shirt, Mapfumo performed with a casual dignity, eventually cracking a smile as the audience keyed into the groove. As exiles will do, he focused on his work, making sure that the show achieved a high standard while keeping his thoughts to himself.