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It has been generally thought that a tiny freshwater fish, medaka (Oryzias latipes), has expanded its habitat into the Japanese archipelago as wet-rice cultivation spread across the region, and hence the distribution of medaka should be a matter of anthropological interest. However, there has been no study to verify this popular belief. To address the issue, we sampled wild medaka, and undertook genetic analyses of its populations. We collected 976 individual medaka from 13 local wild sites, which included 11 paddy-field irrigation channels and two ponds. The gene tree constructed based on nucleotide sequences of mitochondrial genome D-loop region showed no discrepancy in the topology, which is presumed to reflect the northern Kyushu origin of the southern Japanese medaka. Population genetic statistics indicated that the medaka populations in west Japan have greater genetic diversity (heterozygosity) than those in east Japan, supporting the hypothesis that the medaka originate from northern Kyushu. Hence, we argue that the current medaka distribution can be attributed to their past migration event(s) following the expansion of paddy fields from northern Kyushu to the eastern part of the Japanese archipelago.