The influence of the nematode Gammarinema gammari on survival, mating success, and fecundity of its host Gammarus insensibilis (Amphipoda) was investigated. The prevalence and the mean intensity of G. gammari were significantly higher in males than in females. There was a positive and significant relationship between the mean number of G. gammari and male body size, suggesting that accumulation of this parasite had no significant effect on the survival of its host. Males that harbored the metacercariae of the trematode Microphallus papillorobustus had a lower number of nematodes than those that did not harbor the trematode. Fecundity was significantly reduced in infected females, and unpaired females tended to be more frequently infected by G. gammari than paired ones. However, we found no evidence for parasite-mediated reduction in male competitiveness because the mean number of G. gammari was not significantly different between paired and unpaired males.