In the alpine valley of Valtellina there are two Robertsonian chromosomal races of house mouse, the Poschiavo (POS: 2n = 24-26) characterized by metacentric 8.12 and acrocentrics 2 and 10 and the Upper Valtellina (UV: 2n = 22-24) characterized by metacentrics 2.8 and 10.12. The races inhabit separate villages in the valley except in Sommacologna and Sondalo, where they both occur together with hybrids. A total of 179 mice from 16 villages were typed at 13 microsatellite loci. Seven of these loci were localized close to the centromeres of chromosomes 10 and 12, with the prediction that these regions on the race-specific chromosomes would be the most likely to experience a barrier to gene flow. The remaining six loci were localized at the telomeres of chromosomes 10 and 12 and at the centromeres of chromosomes that do not differ between the races. Substantial differences in allelic frequencies were found between the villages with POS and UV races at five of the loci at the centromeres of chromosomes 10 and 12 but at none of the other loci. These differences were not found to distinguish the two races in Sommacologna and Sondalo. Therefore, the centromeric regions of race-specific chromosomes do appear to experience a barrier to gene flow, although this can break down under intense interbreeding between the races. These results are considered in the context of Harrison's (1990) concept of the semipermeability of hybrid zones to gene exchange and in relation to parapatric speciation.