Restriction analysis shows that wild Scandinavian mice belonging to the species Mus musculus contain the mitochondrial DNA of a neighboring species, M. domesticus. This demonstration results from comparisons of Scandinavian mice with authentic M. domesticus and M. musculus from other parts of Europe. Electrophoretic and immunological analysis of eight diagnostic proteins confirms that mice from north of the hybrid zone in Denmark are M. musculus in regard to their nuclear genes. In contrast, the mice tested from this region and a nearby part of Sweden have exclusively M. domesticus types of mitochondrial DNA. Phylogenetic analysis of the restriction maps suggests that the mitochondrial DNAs found in Scandinavian M. musculus could stem from a single M. domesticus female.