Ecological studies have demonstrated that parasites are capable of influencing various aspects of host life history and can play an important role in the structure of animal populations. We investigated the influence of infection by castrating trematodes on the reproduction, survival and population density of the intertidal snail Zeacumantus subcarinatus, using both laboratory and field studies. The results demonstrate a highly significant reduction in the reproductive output in heavily infected populations compared to populations with low trematode prevalence. A long-term laboratory study showed reduced survival of infected snails compared to uninfected specimens, for snails held at 18 and 25°C. Furthermore, parasite-induced mortality in the field was inferred from a reduction in prevalence of infection among larger size classes, indicating that infected individuals disappear from the population, although the effect of parasites varied between localities. A field survey from 13 localities including 2897 snails demonstrated that prevalence of castrating trematodes had a significant negative effect on both population density and biomass of Z. subcarinatus. This study provides one of the first demonstrations of population-level effects of parasites on their hosts in the field. The results of this study emphasise the importance of castrating parasites as potential agents of population regulation in host species with limited dispersal ability.