Two species belonging to genus Trithyreus have been recorded from Japan until now. The present paper deals with the description of Trithyreus daitoensis sp. n. In general appearence, it resembles Trithyreus sawadai KISHIDA known from the Bonin Islands (or the Ogasawara Islands), but can be distinguished from T. sawadai and all the other “Trithyreus” hitherto known by the shape and setational pattern of flagellum of both the sexes.
