Somatic chromosomes of 17 northern birch mice, Sicista betulina, originating from lowland and Tatra Mountain populations were studied. In the whole studied material constant diploid number of chromosomes (2n = 32) was found. Polymorphism of a pair of large-sized autosomes was found; acrocentric, subtelocentric and submetacentric chromosomes in homozygous or heterozygous state form the polymorphic pair. Consequently, chromosome arm number (NF) varies within 60 and 62. In this regard Polish populations of northern birch mouse differ from those of more eastward distribution (NF = 63–64).