The costs of parasitism to host reproduction can be best assessed using field studies to determine overall mating success and experimental studies to examine how parasites may affect mating behavior. We compared the influence of two parasites, Polymorphus paradoxus and P. marilis (Acanthocephala), on the pairing success of their intermediate host (Gammarus lacustris, Crustacea) in both the field and laboratory. Parasitism significantly lowered the pairing success of male gammarids. In the field, P. paradoxus-infected males paired significantly less often than P. marilis-infected or uninfected males. Those infected by P. marilis were also found in precopula significandy less often than uninfected ones. In the laboratory, the pairing success of males infected by either parasite was significantly reduced in both competitive and noncompetitive situations. As in the field studies, the pairing success of P. paradoxus-infected males was significantly lower than that of P. marilis-infected and uninfected males. Polymorphus marilis-infected males were also outcompeted by uninfected individuals, however, their pairing success improved when alone with a female (noncompetitive experiments). We relate the differential influence of the two parasites on the pairing success of male gammarids to their effects on the physiology and behavior of G. lacustris.