The genetic relationship of American plains bison (Bison bison bison) and wood bison (B. b. athabascae) was quantified and compared with that among breeds and subspecies of cattle. Plains bison from nine herds (N=136), wood bison from three herds (N=65), taurine cattle (Bos taurus taurus) from fourteen breeds (N=244), and indicine cattle (B. t. indicus) from two breeds (N=53) were genotyped for 29 polymorphic microsatellite loci. Bayesian cluster analyses indicate three groups, two of which are plains bison and one of which is wood bison with some admixture, and genetic distances do not show plains bison and wood bison as distinct groups. Differentiation of wood bison and plains bison is also significantly less than that of cattle breeds and subspecies. These and other genetic data and historical interbreeding of bison do not support recognition of extant plains bison and wood bison as phylogenetically distinct subspecies.