The effect of the protostrongylid nematode, Elaphostrongylus rangiferi, on the fecundity and survival of an intermediate snail host, Arianta arbustorum, was studied. Over an experimental period of 12 weeks, infected snails had a significantly lower egg production than uninfected control snails. No significant difference in survival was found between infected and uninfected adult snails. Infected juvenile snails showed a consistent decrease in survival with increasing mean density of parasites. A linear relationship was found between snail instantaneous death rate over a 28–day interval and the mean number of parasites/snail. When calculated for 4–day intervals, the instantaneous death rate was highest between 8 and 12 days post–infection. It is concluded that the most pathogenic phase of this parasite is during the first moult.