Molecular phylogenetic analyses of combined mitochondrial DNA sequences (2814 bp; cytochrome b gene, displacement loop region, and NADH dehydrogenase subunit 2 gene) identified nine groups among 49 individual Japanese martens, Martes melampus, collected from several areas in Japan. The grouping was not correlated with winter coat color, but was consistent with geography. In particular, the monophyly of 29 Tsushima martens, M. m. tsuensis, was supported by strong clade support and topological tests. Haplotype and nucleotide diversities were much lower for the Tsushima population than for any population on the Japanese main islands. In addition, analyses of heterozygosity in nuclear growth hormone receptor gene sequences (654 bp) showed genetic homogeneity for the Tsushima population. This evidence supports the view that the Tsushima marten's long history of isolation on small islands is responsible for its genetic distinctiveness and uniformity, validating the Tsushima population as an evolutionarily significant unit.