Plymouth and the Yealm Estuary are situated on the south coast of Devon. The Plymouth area includes a complex of marine inlets, the largest of which is the Tamar which is tidal for about 30 km and is joined by the Rivers Tavy and Lynher. The Tamar empties into Plymouth Sound which is an open bay with an artificial breakwater at its entrance and Drake's Island to the northwest. The River Plym empties into the Northeast of Plymouth Sound. The Yealm Estuary is situated to the east of Plymouth Sound and is tidal for 6.5 km. Plymouth Sound is an important commercial harbour, and the Devonport Royal Naval Dockyard is situated in the lower Tamar Estuary. The area has a long history of marine biological study, both for research and education. Staff from the Marine Biological Association at Plymouth have carried out research in the area since 1884. Research is also carried out by staff at the Institute for Marine Environmental Research, and Plymouth Polytechnic uses the area for student projects and practicals. The coastline of Plymouth Sound is steeply sloping and almost entirely rocky. The coastlines of its associated estuaries are generally more gently sloping, and contain large expanses of mud flats. Rocky outcrops are also present and steep rock slopes occur on some river bends. The Yealm Estuary is a narrow steep sided inlet with some branches. Underwater a steep sided channel winds a course through Plymouth Sound from the entrance to the Tamar and out past the breakwater. Depths of up to 40 m are present in this channel. The Tamar is the main receiver of freshwater inputs and salinity does not normally reach fully marine conditions until it enters Plymouth Sound. The northern area of Plymouth Sound is influenced by this freshwater input. Freshwater input to the Plym and the Yealm is low. Areas of strong tidal streams are present at the narrow rocky entrance to the Tamar and across the bridge between Drake's Island and the western side of the sound. The inner area of the sound is sheltered by the breakwater at its entrance, but the outer sound, particularly on the eastern coast, is subject to strong wave action from prevailing south-westerly winds. The Yealm is sheltered by a sandbar at its entrance. The present survey aimed to collect further information on the marine habitats and communities which had not previously been described or had not been sampled since the early 1900's. The abundance of species living on hard surfaces and visible on the surface of sediments was recorded in situ. Intertidal sediment fauna was collected in 0.01 and 0.1 m2 cores and by digging, and samples were sieved over 0.5 mm, 1 mm and 5 mm mesh sieves. Subtidal sediments were sampled using a pipe dredge and, outside the breakwater, with a suction sampler. The samples were sieved over a 1 mm mesh. Animals were picked out of the sediments, identified and counted. Photographs were taken to illustrate habitats, communities and species. A series of aerial photographs was taken during low water spring tide level in April. Habitats and communities present were classified into 29 intertidal types (22 in the Plymouth area and 7 in the Yealm) and 28 subtidal types (22 in the Plymouth area and 6 in the Yealm). Descriptions are given of each of these and the abundance of species is shown in the tables. The distribution of habitats is discussed and the subtidal substratum types are listed. Nine major ecological zones are suggested, and described, for the Tamar and Plymouth Sound, on the basis of salinity, wave exposure and the communities and species found in them. These zones are termed-Open Sea, Open Coast, Sheltered Bay, Outer Estuarine, Lower Estuarine, Central Estuarine, Upper Estuarine, Riverine-Estuarine Transition and Riverine. The habitats and communities surveyed are compared with those in other marine inlets and with previous marine biological studies in the area. The area of Plymouth Sound and the Tamar is much more extensive than most other marine inlets studied although it includes many similar habitats, communities and species. The Yealm is small but compares very well with other inlets. The main characteristics of the Plymouth area and Yealm estuary are much as previously described in the Plymouth Marine Fauna (1957) and published work from the first half of this century (and late nineteenth century). For some areas our assessments rely on some part on this previous work. However, this survey has found a number of changes, and communities on rocky sublittoral habitats have been adequately described for the first time. There have also been a number of species that have been recorded for the first time. These include species of the red algae, Schmitzia, and a variety of polychaete worms, including Pisione remota, Ampharete lindstroemi, and Protodorvillea kefersteini. The seapen Virgularia mirabilis was also recorded from north of the breakwater, where it has not previously been seen. Apoglossum ruscifolium was commonly recorded in the Hamoaze on this survey but not by Johnson in 1890. The distribution of Fucus vesiculosis in the Tamar appears to have extended northwards since the records of Percival (1929), while that of Cordylophora lacustris appears to have extended downriver. The former suggesting a reduced freshwater input, but the latter an increase. A number of minor changes were also noted in Kithching's gulley outside the entrance to the Yealm. The scientific interest and nature conservation importance of the area has been assessed using standard criteria and the conservation importance of the habitats and communities in the area have been provisionally graded as of Local, Regional, national or International importance. Species of particular scientific interest identified during the survey have been noted and their conservation importance provisionally graded as of Regional, National or International importance. For the general assessment of scientific interest and nature conservation importance the area has been considered in six separate parts:- The Open Sea and Open Coast which is most notable for the Mediterranean-Atlantic species found in deep water offshore; Outer Plymouth Sound which has some interesting rocky shore habitats, particularly the rockpools, and large areas of very rich and interesting subtidal sediment communities; Inner Plymouth Sound which has some very unusual limestone bedrock communities characteristic of rias and with interesting boring species; the Cattewater and Plym Estuary which have some rich mudflats but are generally of lesser importance; the Tamar and Lynher Estuaries which are most notable for the presence of rocky substrata extending along the estuarine gradient and the rich current swept cobbles and pebbles in the lower estuary; and the Yealm Estuary which is of outstanding interest due mainly to the high diversity of habitats and communities which are almost entirely natural. Much of Plymouth Sound and its associated estuaries are affected by human activity, but in the Yealm Estuary current levels of recreational activity do not appear to be damaging its naturalness, except for the presumably high levels of organotin from antifouling paints. Records currently considered sensitive have been removed from this dataset.