Two alleles, ToA and ToB, determine a genetic polymorphism of tetrazolium oxidase in canine erythrocytes. The approximate gene frequencies for most dog breeds are 0.94 for ToA and 0.06 for ToB. The gene frequencies of the German shepherd, 0.8 for To A and 0.2 for ToB, differ significantly from those of other breeds. The electrophoretic tetrazolium oxidase isozyme pattern of Canis Iatrans closely resembles the tetrazolium oxidase A pattern of Canis familiaris.