In this paper a comparative analysis of the two vole species with different type of distribution live in sympatry in southern Siberia and northern Kazakhstan: the narrow-headed vole – a widespread species inhabiting an open ecotopes, and Strelzow’s mountain vole inhabiting rocky outcrops, is presented. The material on their demographic structure and fertility is discussed. The differences in the ecological strategy (growth, development and reproduction) of the species were presented.