Proterometra macrostoma and P. edneyi infect the same snail host, Goniobasis semicarinata, but different fish hosts in their life-cycles. Cercariae of P. macrostoma complete development in sunfish, those of P. edneyi in darters; fish become infected when they ingest free-swimming cercariae as ‘prey’. Laboratory and field experiments were designed to test the hypothesis that light: dark (L: D) cycling regulates emergence of both species. Under L: D cycling conditions, P. macrostoma emerged in the dark and P. edneyi in the light. These emergence patterns resulted from differential sensitivity to light and dark. In the laboratory and field, reversing the light and dark periods resulted in corresponding alterations in emergence patterns of both species. Both species emerged in constant light and constant dark, but their emergence patterns were altered. Emergence patterns may represent adaptations that make the cercariae more susceptible to ‘predation’ by their respective fish hosts.