AbstractAdults of Cyclothyas siskiyouensis n. sp., the first species of the genus reported from the Western Hemisphere, are described from female specimens collected from a small stream in Oregon, U.S.A. Members of this species differ from those of previously described species in having 3, rather than numerous, pairs of genital acetabula along outer edges of the genital flaps. A revised diagnosis of the genus Cyclothyas is presented.