ABSTRACT: Terrestrial herbivores can use anthelmintic properties of plant secondary metabolites to decrease parasite/parasitoid infections. Secondary metabolites with potential anthelmintic properties have also been described from different seaweed species, but the effects of diet on parasitism in marine herbivores has not been studied previously. In the present study, a combination of laboratory and field experiments was used to test the hypotheses that infection by digenean trematode parasites affects the feeding pattern of herbivorous gastropods (Littorina littorea), and that the seaweed diet prevents parasitism in the snails. Heavily infected snails consumed less of the red seaweed Osmundea ramosissima than uninfected snails, but there was no difference in consumption when snails were offered the green seaweed Ulva lactuca. Healthy snails placed in cages with different food treatments and transplanted to different field sites became infected by trematodes, but there was no significant difference in parasite resistance between snails offered different foods. Furthermore, there was no significant negative relationship between infection abundance and consumption when O. ramosissima was incorporated into agar-based artificial diets, indicating that the difference in feeding pattern found in the initial consumption experiment was due to the morphology, and not the chemistry of the seaweeds. In conclusion, these preliminary results do not indicate that parasitized L. littorea feeds on O. ramosissima to prevent parasite infection. However, the hypothesis that algal chemical compounds prevent or decrease parasite infections in herbivores should be tested using additional candidate species before drawing general conclusions about the effect of diet on parasitism in marine systems.