Differences in the parasite faunas of wild and farmed populations of the northern scallops, Argopecten purpuratus, from northern Chile, were compared. Infections of tapeworm larvae (plerocercoids of Acanthobothrium sp. and Rhinebothrium sp.) do not occur in the farmed scallops owing to unsuitable conditions for transmission, whereas, by contrast, there is a significantly higher occurrence of the burrowing polychaeta Polydora sp. in farmed conditions. These results can be explained in that farming conditions preclude completion of the tapeworm life cycle, since the infective stage cannot access the scallops suspended in the water column. Meanwhile crowding in the farmed population, as compared to the wild one, may favor infection by Polydora sp.