Gene-pool divergence between the two sibling vc,le species, Microtus arvalis (2n=46) and M. rossiaemeridionalis (2n=56) was studied by horizontal starch gel electrophoresis of 23 enzyme-systems encoded by 29 structural gene loci. Five local samples of M. arvalis, encompassing three nominal subspecies and one sample of M. rossiaemeridionalis, were studied. M. guentheri (2n=56), a closely related species, was used for comparison. Genetic distances between the two sibling species (N e i's 1978 unbiased D: mean=0.070, range=0.037-0.137) were generally rather low as compared to the values of intra-specific comparisons of M. arvalis (mean=0.039, range=0.005-0.094), the values calculated between M. arvalis and M. guentheri (mean=0.243, range 0.225-0.284) and those between M. rossiaemeridionalis and M, guentheri (0.184). Also, the degree of partitioning of genetic variability was lower between the two sibling species (F-ST=0.342) than within M. arvalis (F-ST=0.425 for the local samples, and F-ST=0.450 for the three subspecies studied). This contrasts the pattern of karyotypic differences (particularly the diploid chromosome numbers) encountered earlier in the three species. This suggests independence of genic and chromosome evolution in the studied species.