Medusoides gen. nov. is described from North Carolina, U.S.A. The fungus reproduces sexually, with oogonia and antheridia, but no asexual stage has yet been found. The fungus has not yet been obtained in culture. The vegetative growth is mycelial but the hyphae are very narrow (<3 μm) with vesiculate nodes. The oospores are strictly plerotic. Oogonial and oospore walls are distinctive with unique ultrastructure. The peronosporomycete is a typical aero-aquatic fungus, having markedly buoyant propagules, due to the possession of long, recurved, oogonial outgrowths covering the oogonium sufficiently densely so as to retain air between them and the oogonial surface after exposure and reimmersion. Its habitat, in the lower layers of submerged deciduous leaf litter, suggests survival in an anaerobic environment and a fermentative capacity. An affinity with Pythiogeton is proposed, and Pythiogeton is made the type genus of a new family, the Pythiogetonaceae.