Studies on the biology and life history of nemerteans are scarce, mostly because these animals are nocturnal. In order to broaden the knowledge base on the life history of nemerteans as a prerequisite for comparative analyses, we studied a population of Riseriellus occultus (Heteronemertea: Lineidae) inhabiting the rocky intertidal in southern Brittany near Concarneau (France) for more than 10 years. Our studies show that R. occultus is an iteroparous, perennial species exclusively inhabiting rocky shore crevices that result from onionskin weathering of the granite. From September through October R. occultus reproduces by external fertilization and develops via a planktonic pilidium larva, which, under laboratory conditions, metamorphoses after about six weeks. Adults of R. occultus are nocturnal macrophagous predators that preferentially feed on the gastropods Gibbula umbilicalis and Patella species, but also consume the bivalve Mytilus edulis. Since R. occultus devours the snail inside the shell, we fixed individuals while feeding, and serially sectioned them. Reconstruction of the sections shows that R. occultus swallows the entire soft body and finally detaches the columellar muscle from the shell. Estimates on the density of R. occultus inside the rock crevices provide evidence for clustered distribution and locally high abundance on the rocky shore. These data strongly suggest that R. occultus affects the structure of the rocky shore gastropod community. Although our data are still fragmentary with respect to the ecology of this species and its role in the local food web, our knowledge has grown to such extent that R. occultus can now be regarded as one of the few well characterized nemertean species.