In mammals, sex determination is controlled by the Y-linked gene SRY. Although SRY is male-specific in most eutherian and marsupial species, with a single copy on the Y chromosome, several rodent species have multiple Y-linked copies of SRY, and two mole-vole species of the genus Ellobius determine sex without the Y chromosome or the SRY gene. We searched for homologs of SRY in three vole species of the genus Microtus and concluded that this gene is not male-specific in M. cabrerae, as it is present in multiple, polymorphic copies in both males and females. In contrast, SRY is male-specific in the related species M. agrestis and M. nivalis. Up to 15 different partial sequences of the SRY gene were found in M. cabrerae. Southern blots suggest that most of the extra copies of SRY are X-linked. One of the copies observed only in males has a sequence identical to that of the SRY gene in M. agrestis and may represent a functional copy of the gene in this species. The rest are probably nonfunctional pseudogenes.