The marine gastropod Trichotropis cancellata, previously considered to be exclusively a suspension feeder, is also a kleptoparasite, stealing food from several species of suspension-feeding polychaetes. When feeding independently, T. cancellata uses its pseudoproboscis, an elongate, ciliated extension of the lower lip, to transport particles captured on its ctenidium to its mouth. When parasitizing, the snail positions its pseudoproboscis in the mouth of a host polychaete and diverts a large proportion of the particles captured by the polychaete to its own mouth. In subtidal habitats around San Juan Island, Washington, most individuals of T. cancellata are found in association with the tube openings of suspension-feeding polychaetes. In laboratory experiments, parasitism significantly enhanced fitness in T. cancellata. Juvenile snails that parasitized polychaetes grew faster and survived in greater numbers than those deprived of access to hosts, and parasitic adult snails reproduced more than those without hosts. Parasitism in T. cancellata and related capulid gastropods may have originated in early post-metamorphic stages as a response to constraints on the efficiency of suspension feeding at small sizes; however, because parasitism is more effective than suspension feeding for snails of all sizes, it now persists throughout life.