Loch Fyne, which opens from the north of the Firth of Clyde, is both the longest of the Scottish sea lochs, at approximately 70 km, and the deepest, with a maximum charted depth of 200 m. The large size, and corresponding large volume, result in the water exchange time of 13 days being the second longest of any Scottish sea loch. The two sills, at Otter and Minard, place the loch in the category of a type 'C' fiordic loch (Milne 1972). Because the loch opens on the relatively sheltered Firth of Clyde, it is not exposed to much wave action even at its mouth, and the normal transition from open coast to extreme shelter seen along other sea lochs is not apparent. The loch is generally steep sided, particularly in the upper half, and the intertidal area is relatively small, and almost entirely rocky. Bedrock and boulders extend into the infralittoral throughout most of the loch, whilst circalittoral rock is predominantly found in the upper loch. Sublittoral sediments in depths of less than 50 m are generally a mixture of mud or sand and coarser components, with little evidence of distribution controlled by wave exposure. The soft mud that has been reported by previous studies to lie in the basins of the loch, was not surveyed during the present study because of the depth limitations of SCUBA diving. There are numerous centres of population around the loch, although most are small. Ardrishaig, in Loch Gilp, is at one end of the Crinan Canal, which allows boats protected access from the Clyde to the outer coast, and so is the focus for considerable boating activity. Tarbert, in the lower loch, is a fishing harbour with a fishing fleet that works predominantly in the Firth of Clyde. Loch Fyne itself is not a regular commercial fishing ground. There are a number of fish farms of different types around the loch, and several more sites have licences for fish farm development. The upper loch includes several very popular dive sites, used mainly by local diving clubs. Two SSSI's border the western shoreline of the lower loch, and there are other sites of conservation interest with, as yet, no official status at Otter Spit, Kilfinan Bay, Auchalick Bay and Glenan Bay. The upper loch north of Creag a'Phuill has been made a Marine Consultation Area on the strength of this survey. Loch Fyne has a long history of faunal recording, extending back over the last 200 years, and numerous other scientific studies have been published on aspects of marine biology within the loch. The present survey, carried out in August and September 1988, aimed to describe the marine habitats and communities present in the loch and assess their nature conservation importance. A total of 17 littoral and 44 sublittoral sites were visited, the latter being surveyed by diving. At each site the abundance of the epiflora and fauna was recorded and the habitats described. No fully quantitative infaunal sampling was carried out. Twelve littoral and 12 sublittoral habitat types were identified and these and their associated communities are described in this report. A list of the taxa recorded is also given. A rather limited range of communities and habitats was found in the loch, ranging from extremely sheltered types near the head of the loch to sheltered communities in the rest of the loch. No open coast communities were found, but a distinctive community was located in the central narrows, where tidal streams are accelerated. Otter Spit, an interesting geological feature, proved to support communities that could, in the main, also be found on mixed sediments in embayments elsewhere in the loch. Sea pen beds and megafaunal burrowing communities, both typically found in the sheltered waters of sea lochs, were not located during the survey. They are probably present in deeper water in the loch. The scientific interest and conservation importance of the loch have been assessed using standard criteria. Five communities and 36 species have been provisionally assessed to be of regional or international conservation importance. Although Loch Fyne lacks the diversity of habitats and communities seen in some sea lochs, it holds good examples of sheltered circalittoral cliffs, and the head of the loch supports exceptional densities of the cerianthid anemone Pachycerianthus multiplicatus. The latter species is very restricted in its geographical distribution, and so confers considerable conservation importance on the upper loch. It is quite possible that the deep water of the loch also holds unusual communities, since there are very few places where such great depths are present in close proximity to land. In an attempt to gain information on these deeper, unsurveyed, parts of the loch, a second survey was carried out by means of a video camera mounted on a sledge. This enabled information to be collected on the types of habitat and community present in the deeper sediment-covered areas of the loch and their extent to be assessed. It also provided an opportunity to assess the benefits and limitations of the technique for this type of survey work. The survey was carried out over six days in January and February 1990 from the UMBSM R.V. 'Aora'. The intention was to carry out a series of tows which together covered the entire midline of the loch but in practice this sequence was broken by areas in which the use of towed gear was not feasible. Thirteen tows were carried out over twelve sections of the loch, covering 59 km of seabed in 24.5 hours of recording. Densities of burrow openings, Pachycerianthus multiplicatus and Virgularia mirablis were estimated over short timed sections of tape where seabed visibility permitted. Thirteen habitat and community types were described. The upper loch was filled with large areas of heavily burrowed mu in depths of 26 to 138 m with little apparent anthropogenic disturbance. The thalassinid shrimp Calocaris macandreae was the major burrowing species with the Norway lobster Nephrops norvegicus scarce or absent. Both the echiuran Maxmuelleria lankasteri and the anenome Pachycerianthus multiplicatus were common. A bed of the scallop Pseudamussium septemradiatum was found. The central section of the loch contained sandier muds, also burrowed by C.macandreae but with more N.norvegicus and the sea pen Virgularia mirablis. The loch entrance reaches depths of 186m. The mud here is heavily trawled for Norway lobsters and much trawl debris was seen on the video. Shelves at the sides of the entrance basin have rocky crags which preclude trawling. Burrows here were numerous with both C.macandreae and N.norvegicus common. Sandier sediment on the Lamont Shelf held a bed of the thalassinid Callianassa subterranea and the echiuran Amalosoma eddystonense. Harder ground was found around Otter and Minard narrows where brittle stars were abundant in the stronger tidal streams. Reefs of boulders and bedrock were found in depths of 56 to 75 m in all three sections of the loch and large numbers of the anenome Bolocera tuediae were found on these. Small numbers of the northern spider crab Lithodes maia were found throughout the loch. A comparison of the results was made with those from the earlier survey, faunal records in Chumley (1918) and observations from a manned submersible made by Eden et al. (1971). Some detrimental effects of trawling for Norway lobster were also noted. The towed video technique was found to be a useful additional tool for the type of survey being carried out by the MNCR as it supplements data from remote sediment sampling with visual information. However, it is not an effective method of surveying rocky ground and the deep rock outcrops in Loch Fyne remain to be surveyed. Three species and nine habitat/community types were provisionally assessed to be of Regional, National or International conservation importance. Records currently considered sensitive have been removed from this dataset.