Loch Cairnbawn is one of the most northerly of Scottish sea lochs, only Lochs Laxford, Dughaill and Inchard lying further north on the west coast. The Y'-shaped loch system, comprising Lochs a' Chairn Bhain, Glencoul, Glendhu, Beag and Shark, is 15 km long and reaches maximum depths of 111 m in the deepest of its six basins. The relatively complex series of basins and sills categorize the system as a typeC' (Milne 1972), and result in the absence of the seasonal fjordic loch stagnation of deep water shown by other fjordic lochs. The system is steep-sided, with a relatively small littoral area, bedrock and boulders extending through the littoral and into the circalittoral in almost all areas. Sediment predominates in the middle of the loch. The sediment composition depends heavily upon the level of exposure to water movement with coarse, clean sediments in Eddrachillis Bay grading to fine soft muds in the deep water basins and at the loch heads. The sequence of sediments is interupted in the narrows region at Kylesku and on the sills, where increased current speed results in the presence of coarser sediments. The area in which the loch lies is fairly remote, with little habitation and no shore-based industry. Major uses of the loch are scampi fishing, by trawl and creel, and fish farming. The number of leased fish farm sites is currently at least 12, not all of which were occupied by fish farms at the time of the survey. Two Sites of Special Scientific Interest border on Loch Cairnbawn, one at the head of Loch Glencoul, and the other along most of the southern shore of Loch a'Chairn Bhain. Very few marine biological studies have been carried out in the loch, these being limited to some littoral recording and some grab sampling of sublittoral sediments. The present survey, carried out in October 1988, aimed to describe the marine habitats and communities present in the loch and to assess their nature conservation importance. A total of 40 sites were visited, of which 4 were littoral and 36 were sublittoral. At each site the abundance of the epiflora and fauna was recorded and the habitats described. No infaunal sampling was carried out. Thirty two sublittoral and three littoral habitat types were identified, and these are described in this report. A list of the taxa recorded is also given. The communities present in Eddrachillis Bay were open coast in nature, and there was a transition to sheltered sea loch communities further into the system. The narrows area in the middle of the loch an exception to the gradual transition, with the presence of a range of high energy habitats due to the accelerated tidal streams in this region, and was one of the more interesting features of the system. The scientific interest and conservation importance of the area have been assessed using standard criteria. These have been applied to each of the habitats or community types identified which have been provisionally graded as of Local, Regional, National or International importance. Species of particular scientific and conservation interest have been provisionally graded as of provisional Regional or National importance. Loch Cairnbawn appears to contain a good range of the habitats and community types typical of sheltered, fjordic sea lochs, with no particularly unique features. The full conservation significance of communities in the loch will only be adequately assessed when surveys of other Scottish lochs are complete. Records currently considered sensitive have been removed from this dataset.