Mussels are found in two quite different kinds of habitat between tide marks: open coast, and very sheltered places such as harbours and estuaries. This is true also of the Lough Ine area (Ebling, Kitching, Sleigh & Sloane, in press). Here the open sea Mytilus are all rather small, whereas those in shelter at the north end of the lough and in the Goleen include many large ones. Mytilus is present both in the straight (M. edulis) and hooked (M. galloprovincialis) forms (Hepper 1957) and both are present in both types of habitat (Ebling et al., in press). We have also seen Mytilus in a third kind of habitat at Lough Ine, sublittorally in strong current on the Sill. It is possible that the discontinuous distribution of Mytilus in the Lough Ine area is due to predation. On this hypothesis the predators would be affected adversely by wave action, by strong current, and also by some condition (possibly low salinity) operating in those parts of the lough least affected by tidal exchange. Mytilus would also benefit by situations where it was less accessible, such as deep crevices and vertical rock faces. The predacious gastropod Nucella lapillust is abundant on the dense beds of small Mytilus at open sea stations and is also found on sublittoral Mytilus on the Sill. Other possible predators of established mussels include the crabs Cancer pfagurus, Xantho incisus, Carcinus maenas and Portunits puber, and the starfishes Asterias rubens, Marthasterias glacialis and Asterina gibbosa, all common in the lough. The sea urchin Paracentrotus lividuts is abundant in the lough, and Echinus esculentus is not infrequent from the Bullock Narrows to the south-east corner of the lough. Of these Echinus is known to include animal material in its diet. Many other predators may affect the younger stages, so that the problem is likely to be complicated. We have so far confined our attention to Mytilus of the range of sizes found at Carrigathorna on the open coast (up to 25 mm in length) and at the north end of the lough (mainly 30-70 mm). Preliminary samples suggest that the small Mytilus found at Carrigathorna are almost entirely M. edulis and the large ones at the north end of the lough mainly M. galloprovincialis, but further investigations will be needed before we can report definitely on the distribution of these two forms or comment on their validity as species.