The geographical distribution of mtDNA haplotypes of non-indigenous smallmouth bass (Micropterus dolomieu) populations in Japan were examined utilizing nucleotide sequences of the mtDNA control region from 208 individuals collected from 20 localities. A total of three haplotypes (n, o and p), distinguished by a nucleotide substitution or an insertion/deletion of 294bp, were found in Japanese freshwater systems. In eastern Japan, most M. dolomieu populations had two haplotypes (n and p), the frequencies being similar among localities. On the other hand, haplotype n or p was fixed (or nearly fixed) in lakes and ponds in western Japan. These results suggested that the non-indigenous populations of smallmouth bass in Japan were initially established by an introduction into eastern Japan, the western Japanese populations being subsequently founded by relatively small num-bers of individuals.