Six lochs were surveyed by UMBSM/NCC during a visit to Harris and Lewis in 1988 (Howson 1989). Six more were subsequently surveyed in August 1990: Loch Stocknish, East and West Loch Tarbert and Lochs Erisort, Leurbost and Grimshader. Loch Stocknish, on the south-east facing coast of South Harris, is classed as a fjordic B type loch by Milne (1972). It is short (4km), has one sill and its inner basin reaches a depth of approximately 40 m. Most of the loch is sheltered from wave action and there are areas of tide_swept sediment in the narrow entrance channels. East Loch tarbert and the Erisort-Leurbost system are classified ad fjords with fjard-like characteristics (Fairbridge 1968). Both have a number of islands protecting the inner loch from wave action and consequently a large range of exposures were encountered in the littoral and sublittoral. East Loch Tarbert has a deep basin (66 m) situated to the north of Scotasay and a tide-swept narrows between Scalpay and South Harris. This loch has extensive areas of shelly sediment which at the time of the survey were being dredged for scallops. Softer muds were found in Loch Ceann Dibig near the head of East Loch Tarbert. Loch Erisort is the longest loch in this series (12 km). It reaches a depth of 56 m near the mouth, but is shallow behind its main sill. It has areas of littoral and sublittoral exposed bedrock but, with Loch Leurbost, was also noted for having a wide range of sediments. At the mouth of Loch Erisort, concealed behind a group of islands, there is a complex of shallow, sheltered channels and small lochs, some of which are brackish. Loch Grimshader is a small (2.5 km) B class fjord (Milne 1972) although it has been described as an ob by Earll and Pagett (1984). It has a single basin (maximum depth 13 m), a narrow, shallow, tide-swept entrance and steep bedrock slopes just outside the entrance. West Loch Tarbert is very different in character from the east coast lochs. It is a large open bay, rarely deeper than 30 m and is very exposed to the west. There are no sills, although there is an area of deep water (50 m) in the Soay Sound. The sediments are predominantly muddy sand. There are a considerable number of fish farms throughout these lochs, particularly in Lochs Erisort and Leurbost, and evidence of nutrient overloading was found in localised areas with poor water circulation in Lochs Stocknish, Erisort and Grimshader. There is one SSSI adjoining the north shores of West Loch Tarbert, and the outer region of Loch Erisort is rated as of marine biological importance. Much littoral work has been carried out by Smith (1979, 1982, 1983, 1984) and Powell et al. (1979) but there have been few sublittoral studies in this area. In the present survey seventy nine sites were surveyed, of which 61 were sublittoral (4 where suction samples were taken) and 18 were littoral. Thirty two habitat/community types are described. Infralittoral bedrock communities ranged from rich Laminaria hyperborea forest in the wave and tide exposed areas to dens L.saccharina in the most sheltered regions. In West Loch Tarbert the kelp forest communities were heavily grazed and the deeper communities effected by scour. Examples of exposed circalittoral bedrock with Leptometra celtica, Swiftia pallida and Diazona violacea were found at the entrance to the east coast lochs. Loch Erisort held a particularly wide range of sediment communities including the species-rich sill, the shallow and seemingly undisturbed sediment with dense Virgularia mirablis and Philine aperta and the shallow, soft muds with very large Suberites domuncula. A dense bed of Ocnus planci, rarely found in such numbers, was found towards the head of the loch. Seventeen habitat/community types and 17 species have been provisionally assessed to be of Local, Regional or National importance. Records currently considered sensitive have been removed from this dataset.