Polymorphism of kappa-casein gene (alleles A and B) in Belovezhsky and Caucasian-Belovezhsky European bisons, bisons and their hybrids, as well as in cattle, yaks, and Yakut cattle was studied by PCR method, followed by restriction analysis. European bisons and yaks, being referred, correspondingly, to genera Bison and Bos of Bovidae family, were stated to have A and B alleles, previously found in cattle (Bos taurus). Yakut cattle was shown to have a new rare F allele of kappa-casein gene with 7.7% frequency. The comparative genetic analysis of the above groups, according to kappa-casein polymorphism, revealed similarity between Belovezhsky and Caucasian-Belovezhsky european bison strains. Observed decrease of A allele frequency in Caucasian-Belovezhsky European bisons must be induced by the high level of inbreeding. Measures preventing the loss of genetic diversity in lines of european bison was proposed.