We studied variations in genetic, physiological, and ecological traits, and the phylogenetic relationship among sexual and parthenogenetic populations of Asobara japonica, a larval parasitoid of drosophilid flies, in order to understand how they adapt to local environments and have differentiated. The strain from Iriomote-jima (IR) differed from other Japanese strains in the nucleotide sequences of its cytochrome oxidase subunit I (COI) and in not undergoing diapause and having a shorter preimaginal period and a higher adult tolerance of cold. The strains other than IR showed a low level of nucleotide variation in COI but varied in their mode of reproduction; the strains from the Ryukyu Islands were sexual, whereas those from the main islands of Japan and Ogasawara were parthenogenetic. In addition, strains from higher latitudes generally showed a high incidence of diapause, although there were some exceptions. On the other hand, preimaginal period and adult cold tolerance varied little among the strains excluding IR, and pupal cold tolerance, oviposition preference and incidence of parasitism varied little among the strains including IR. Evolution and environmental adaptations in this species are discussed, particularly focusing on parthenogenetic populations.