Studied was the nucleotide composition of 34 strains of gram-positive cocci isolated from slaughtered birds, sheep milk, and milk from mastitis-affected cows and ewes. The differentiation of the strains was carried out in conformity with the instructions of Bergey's Manual of Determinative Bacteriology (1974) on the basis of the following indices: morphology of colonies, morphology and staining of the cells after Gram, catalase, oxydase, respiration type, fermentation of manite, production of plas mocoagulase, thermonuclease, and phosphatase and response to novobiocin. The amount of guanine, adenine, cytosine, and thymine in the investigated strains was determined after the method of Spirin and Belozerskii. On this basis the guanine-cytosine percent was calculated. Results showed that the use of the guanine-cytosine percent made it possible to more accurately differentiate the genus with gram-positive cocci. In 88.24 per cent of the investigated strains there was coincidence between the guanine-cytosine percent and their biochemical characteristics. Two of the strains (guanine-cytosine percent 49.95 and 47.48) were shown to belong to the group of planococci. No typical micrococci (with a guanine-cytosine percent of more than 60.00) were established.