Baseline surveys of Islay and Skye show that the shores of the latter island have the more diverse mollusc population. Detailed investigation of the Mollusca associated with algae corroborates the general survey. The reason for the comparative paucity of Mollusca on Islay appears to stem from a number of factors of which the lack of variety of habitats and the shorter shores with little tidal range are the most obvious. Additional work carried out by diving and dredging enhanced the data from Skye, supporting the view that a combination of approaches produces the best results in a baseline survey, and helps to minimise any false impression of a marine fauna which may arise in surveys arbitrarily restricted to above or below low water mark or to a single method of obtaining data. The relationship of the marine molluscan fauna of Islay and Skye to that of west Scotland as a whole is briefly discussed.