Introductions of black basses Micropterus spp. beyond their native ranges have led to hybridization within the genus. In the southeastern USA, the potential for hybridization appears high because species introductions have been common in reservoirs. We determined the extent of hybridization between smallmouth bass M. dolomieu and spotted bass M. punctulatus in reservoirs in which introductions of either species into the native range of the other species had occurred. Three allozyme loci were used to distinguish the two species and their hybrids. Significant hybridization occurred in two of three reservoirs where introductions had been reported. In Lake Chatuge, Georgia-North Carolina, where the Alabama subspecies of spotted bass M.p. henshalli was introduced, 77 of 276 fish had hybrid genotypes, and only 2 fish had genotypes of the native smallmouth bass. In Thurlow Reservoir, Alabama, where smallmouth bass were introduced and Alabama spotted bass were native, 3 of 17 fish had hybrid genotypes. Only I fish with a possible hybrid genotype was identified in two reservoirs containing native smallmouth bass and northern spotted bass M.p. punctulatus.