The distribution and zonation of Chthamalus stellatus and Balanus balanoides on British coasts is reviewed, and it is shown that the very sheltered waters of Loch Sween and West Loch Tarbert are unusual in that Chthamalus occurs there in greater quantity than in any other sheltered (British) locality, and the upper limit of Chthamalus is consistently higher, relative to other species, than elsewhere in shelter. The lower limit of Chthamalus is generally at a rather high level on these shores even when bare rock occurs below. Balanus balanoides, on the other hand, is often scarce or absent at the same sheltered sites. The physical conditions in the lochs (water temperatures, tides, winds, sunshine, rainfall) and their seasonal variations are examined and it is shown that there probably is a unique succession of environmental factors that would favour Chthamalus and operate against Balanus at critical times of the year. It is suggested that Chthamalus occurs higher than Pelvetia in the lochs because of a greater tolerance of irregular and often prolonged periods of desiccation. The failure of Chthamalus to occupy the middle and lower shore when Balanus is absent indicates that competition between the two barnacles does not alone determine their vertical distribution on the shore. A tentative hypothesis is developed showing how the very local distribution of Chthamalus and the level occupied on the shore might be related to responses of the cyprids to light and to a supposed requirement of emersion during the early stages on the shore.