EN
An electrophoretic study of the variation at 40 protein loci in the bank vole Clethrionomys glareolus {Schreber, 1780) was performed in spring 1994-1996 and in autumn 1994 and 1995. A total of 153 individuals from deciduous forest and 122 from coniferous forest subpopulations were collected. During the whole time of the study in spring the coefficient of trappability was 4.1% in deciduous forest and 3.0% in coniferous forest; in autumn 9.3% and 7.6%, respectively. In spring there were no significant differences in allele frequencies between the subpopulations studied. The samples from deciduous and coniferous forests were genetically similar. However, in autumn these differences were statistically significant. Although the level of heterozygosity in both, neighbouring subpopulations is similar, its changes in consecutive seasons have different values. From the calculations performed, based on the level of genetic differentiation between subpopulations (Fst), it can be concluded that the level of gene flow between the voles from two biotopes in spring is greater than in autumn. We hypothesise that the essence of this phenomenon lies in a non-random fraction of migrants between the biotopes studied. The data presented in the study indicate that the genetic structure in two neighbouring subpopulations of the bank vole undergoes different processes.