Amphidinium is a genus of naked dinoflagellates, characterized by a small epicone and a larger hypocone. Presently about 120 species from both freshwater and marine biotopes have been described. The genus has long been known to be poly-phyletic, however, and the recent rediscovery of the type species has now allowed for more precise definition of the genus. The new circumscription of the genus leaves the freshwater species without a generic name. A new generic name, Prosoaulax gen. nov., is therefore proposed for the first described freshwater species, Amphidinium lacustre Stein (1883), and related species. Based on ultrastructural features, notably the very unusual type of eyespot, Prosoaulax is considered to be related to a recently recognized, but poorly understood group of dinoflagellates, comprising the marine species Gymnodinium simplex, G. natalense, Polarella antarctica, and some symbionts of marine invertebrates, G. bei and G. linucheae. It also includes the symbionts of corals (‘zooxanthellae') belonging to the genus Symbiodinium, a genus believed to contain the modern representatives of the Suessiales, an order of dinoflagellates extending back into the Mesozoic