The azygiid trematode, Proterometra dickermani is shown to complete its life cycle in a single host species, the snail Goniobasis livescens. No vertebrate hosts have been found in nature, and experiments have produced infections in sunfish (Lepomis gibbosus and L. macrochirus) and bass (Micropterus salmoides) on rare occasions only. Cercariae reach full maturity within the germinal sac in the snail host. Embryonated eggs are deposited and miracidia hatch in the germinal sacs. Some of the miracidia develop into primary germinal sacs without leaving the individual host; others escape and enter other snails. It is probable that an occasional cercaria leaves the snail and is ingested by a fish. Two complete cycles occur during a year, with hatching of miracidia in early spring and late summer. The extreme progenesis in this cycle indicates a primitive position of the genus Proterometra.