The role of parasites in the evolution of sex has begun to receive considerable attention in recent years. In this dissertation I examine the effects of digenetic trematodes on the origin and consequences of apomictic parthenogenesis in a freshwater snail. I argue that the metacercariae stage of Leuchochloridiomorpha constantiae may have caused the origin of parthenogensis in the freshwater snail, Campeloma decisum, by the ingestion of sperm, resulting in severe sperm limitation. Electrophoretic analysis suggests that an alternative hypothesis, namely hybridization between genetically-divergent sexual ancestors, was not necessary for the origin of parthenogensis in this snail.