The data on the changes in the cytogenetic structure of the geographic population of Korean field mouse Apodemus ( Alsomys) peninsulae Thomas, 1907 at the southern shore of the Teletskoye Lake (Altai Republic) are presented. In 1980 no dot-like microchromosomes were found in 34 mice captured on the southern and northern coasts of the Teletskoye Lake. In 2011, a 1.6-fold (from 2.7 to 4.3) increase in the mean number of B chromosomes compared to the rate estimated there earlier in 1980 was discovered. In 11 of the 15 mice (73%) captured in 2011, the karyotypes contained 1–2 dot-like micro B chromosomes and 1–5 macro B chromosomes. The pollution of the territory by the remains of the rocket fuel components may be an appropriate explanation for the cause of the karyological changes observed in A. peninsulae in this region.