Polymorphism of nine blood group systems was studied in three reproductive lines of auroch (European bison Bison bonasus), Belovezhskaya, Caucasian-Belovezhskaya, and Gornaya. All blood group systems analyzed were polymorphic; 50 out of the 57 antigens tested were detected. The number of detected blood group antigens in aurochs is comparable to that in breeds of domestic cattle. However, intrapopulation diversity, estimated with the mu test, is significantly higher in cattle than in aurochs. The lowest mu values were observed in line Belovezhskaya, which was founded by a small number of animals and has a high level of inbreeding. For the FV system, the amount by which heterozygotes exceeded the Hardy-Weinberg proportions was shown; this deviation was significant in all lines except Caucasian-Belovezhskaya. The excess of heterozygotes may be due to population processes that prevent the loss of genetic diversity in populations. The level of differentiation in the auroch lines was low. Similarity coefficients between the lines were higher than those between different stocks within a cattle breed.