Field surveys were made for geographic distributions of two geckos, Gekko hokouensis and G. yakuensis, within Yakushima, a mountainous island located in southwest of Kyushu, mainland Japan. Gekko yakuensis was found from an almost entire lowland area encompassing various environments, whereas G. hokouensis was restricted to four villages and their vicinity. In many other villages G. hokouensis was not found at all, although G. yakuensis occurred. The four villages having G. hokouensis populations are characterized by ports that have or once had regular vessel sails from other islands of the northern Ryukyus or southern Kyushu where this species is common. Our results strongly suggest that the current Yakushima populations of G. hokouensis have originated from multiple accidental artificial introductions from outside and that G. yakuensis is the only native gekkonid of the island. In this study, we also found hybrids of G. hokouensis and G. yakuensis from a few localities on Yakushima Island. This suggests that genetic introgression may be in progress from the former to the latter species on this island.