Impacts of the trematode parasite Zoögonus rubellus on individuals of its host, the infaunal sandworm Nereis virens, were explored to assess influences that parasites might have on the structure of soft-bottom infaunal communities. Parasites were common in sandworms collected from a mudflat in Maine, with prevalence reaching 100% for large, commercial-sized sandworms and intensities of infection often exceeding 100 metacercariae per host. Parasitism was determined to be costly as sandworms exposed to parasites in the laboratory had higher mortality and foraged less on a main prey item, the amphipod Corophium volutator, than unexposed controls. As Nereis and Corophium are key intermediate-level members in infaunal communities, it is argued that parasitism of sandworms might directly and indirectly influence the structure of soft-bottom communities.