We identified the HindIII restriction site polymorphism of K-casein in cattle reported by Pinder et al. (Animal Genetics 22, 11, 1991) and found an additonal polymorphism (RsaI) in cattle and bison. The HindIII and RsaI restriction sites were mapped and three haplotypes (alleles) were identified. Preliminary screening of 39 cattle and 71 bison revealed one allele restricted to cattle, one restricted to bison, and one shared by the species. No fixed allelic differences were observed among cattle breeds or among bison herds or subspecies.