Genotypic variability in two Moldovian populations of moundbuilder mice Mus spicilegus Pet. was examined at different stages of their life cycle--at the beginning of overwintering in mounds and in the middle of summer in agrocenoses. Thirty allozymes were assayed. Significant differences in allele frequencies and heterozygosity between overwintered and mice born in the year of the experiment were recorded only for Idh-1. This fact was related to the presence of differential mortality among overwintered mice in the populations by the middle of summer. The increase in heterozygosity for Idh-1 in the populations occurred because of "old" mice that lived until the middle of summer, at which point they still have not contributed to the gene pool of the population. The genetic effect of the annual separation of generations of mound-builder mice due to their overwintering in mounds is analyzed. Overwintering has a bottleneck effect on the population. The relatively low genetic variability in mound-builder-mice populations is considered with regard to their biological and ecological features. Based on obtained data, a conclusion on the high genetic stability of moundbuilder-mice populations during their entire life cycle is made.