The paper concerns the interactions of Mytilus edulis, Nucella lapillus and certain crabs as observed in field experiments in Lough Ine, Ireland. In cages in the shallow sublittoral, Carcinus maenas broke open only small Mytilus, Portunus puber broke open fairly large ones, and Cancer pagurus broke or pried open the largest available. Nucella survived and fed over an experimental period of 5 weeks in cages with Mytilus as food. Nucella from the open coast has a larger body whorl, a larger aperture, and a thinner and weaker shell than Nucella from the Rapids. In cage experiments, Nucella from the open coast proved much more susceptible to damage by Carcinus than those from the Rapids. The method by which Portunus puber feeds on Mytilus and on Nucella is described from observations made in the laboratory.