Following the behavioural alterations induced by the trematode Microphallus papillorobustus Rankin 1940 (Trematoda, Microphallidae) on its second intermediate host, the amphipod Gammmarus insensibilis, infected individuals are likely to mate among themselves. We investigated the influence of parasite intensity on the reproductive biology of infected hosts. In the mating system of amphipods, males compete severely for access to females and large males have greater ability to obtain large and more fecund females. We showed that the null hypothesis of random pair formation according to parasite intensity could not be rejected. In addition, infected males obtained females of the expected size according to their own sizes, whatever their parasite intensities. However, in both males and females, the parasite intensity increased the intermoult duration. Because size and reproductive success are strongly correlated in amphipods, we discuss the influence of this process on host fitness.