A new black-fly species, Simulium nemuroense sp. nov., is described from pharate female, pharate male, pupal and larval specimens collected from Hokkaido, Japan. This new species is assigned to the slossonae speciesgroup within the subgenus Simulium (Simulium), by having the following characters: in both sexes of adults, no hairs on the basal portion of radial vein; in the female, the claw with a large basal tooth and the genital fork with two projections on each arm, one directed anteriorly

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... the other posteromedially; in the male, the ventral plate lamellate with a median wide projection, the style longer than the coxite and with a prominent basal protuberance; and in the larva, the postgenal cleft very deep and the presence of the ventral papillae. S. nemuroense sp. nov. is easily distinguished from all the three known species by several characters including the male genitalia with a wide ventral plate and a hairy basal protuberance of the style, and the cocoon with an anterodorsal projection. This is the first record of the slossonae species-group in Japan.