The molluse intermediary host Cernuella (Cernuella) virgata was infected both in nature and in the laboratory with Neostrongylus linearis larvae. The infection rate was higher in the natural infection than in the experimental one all through the study. Accordingly, the survival of the molluscs was lower among those that had been naturally infected than among uninfected control and experimentally infected ones. The differences between survival curves of the different batches were only significant in the months when the infection rate in naturally infected batches was very high, from November to February. So we can conclude that the mortality rate is higher among the snails which harbour a considerable number of N. linearis than among non-infected or moderately infected molluscs.