Blood, kidney and tick samples were obtained from 18 hunter-killed black bears (Ursus americanus) from three sites in northern Wisconsin. A Borrelia sp., morphologically and antigenically similar to Borrelia burgdorferi, was isolated from the blood of two of the animals, and from the kidney of a third. Ixodes dammini and Dermacentor variabilis were found on the bears. This is the first report of borreliosis in the Ursidae, and of the primary vector of Lyme disease, I. dammini, from this host.