This report covers work done under contract to the NCC which was part only of a larger survey of the Isles of Scilly and is therefore far from comprehensive. The remit was limited to descriptions of selected stations and lists of Mollusca found and these data alone are inadequate for the drawing of conclusions regarding the conservation importance of the area, but should be used in conjunction with the results of other surveys. Seventeen stations were surveyed, all habitats investigated, standard samples of Fucus serratus and small algae taken for laboratory analysis of their associated fauna. Mollusca, Polychaeta, Pycnogonida and other fauna were recorded. Most stations investigated were moderately sheltered. The rock of the Scillies is mostly granite, which weathers into angularly jointed rock with gullies and fissures. There were few habitats suitable for species associated with softer rock and/or crevices. Large areas are boulder beds. When covered with algae these support an extensive fauna, but others are somewhat barren. The sandflats are composed of rather mobile granite sand with a small carbonate fraction and scouring of adjacent rock and boulder areas occurs. A few small patches of Zostera were examined. The discussion notes that about 90 species of Mollusca were found. Reasons for this paucity of Mollusca were suggested as follows: Lack of variety of habitat. Large amounts of coarse, unstable sand with lowered infaunal populations and sand scouring and choking of adjacent rocky areas. Lack of carbonate in the sand and absence of finer sediments. Absence of some species due to isolation - larvae in the plankton from mainland coastal areas not reaching the Isles of Scilly. Currents come chiefly from the Atlantic or south-west Ireland and, as well as water movement, the time spent in the plankton may be a barrier to larvae. Past species lists of Mollusca are noted as being very long; it is suggested that the fall-off in Mollusca populations might be due to the considerable reduction in the amount of Zostera in the 1930's with subsequent increase in mobility of the sand. A comparison of Mollusca found during this survey was made with other island areas. Where there was little variety of habitat, the molluscan fauna was broadly comparable, but where stable sediments were present the numbers of species on the shore were much higher. However, comparison of data from all those areas for Mollusca associated with small algae does not indicate marked differences. It is concluded that the nature and variety of habitat, both on the large and small scale, is the most important, and on present knowledge the most easily understood factor governing the diversity of Mollusca in an area. Rich sites were identified - the richest stations were characterised by boulders upon, but not embedded in stable gravel, with shallow pools and running water in moderate exposure. Of the sediment areas, the most sheltered were the richest. Three areas surveyed during this investigation have been the subject of previous reports, and the findings of these reports were briefly discussed. Records currently considered sensitive have been removed from this dataset.