A partial revision is presented of the ascothoracid crustacean genus Ascothorax Djakonov, bursal parasites of ophiuroids. Ascothorax ophioctenis Djakonov parasitic on Ophiocten gracilis in the Norwegian Sea may be specifically distinct from high Arctic A. ophioctenis living in O. sericeum. Ascothorax bulbosus Heegaard from South Georgia is divided into the nominal species infesting Amphiura belgicae, and Ascothorax mortenseni sp.n. in A. microplax. The following species are described: A. octopterus sp.n. in Dougaloplus amphacanthus from California, A. sosci sp.n., host unknown, from the abyssal subantarctic, A. pilocaudatus sp.n. in North Atlantic Ophiosphalma armigerum, A. brattstroemi sp.n. in a Caribbean basket star; 2 other new Atlantic species based on limited material remain unnamed. Females of most species have a pair of filamentary appendages at the base of the first thoracopods, and unlike other ascothoracids, do not have seminal receptacles in the second limbs. There is a problem in distinguishi...