Christchurch Harbour is situated in east Dorset, approximately mid-way between Poole Harbour and the Solent. It was formed during the post glacial marine transgression which drowned the combined lower river valley of the Rivers Avon and Stour. It has a length of approximately 2.5km and an area of not more than 2km2. The harbour consist mainly of intertidal fine muddy sand, with small areas of salt marsh around its southern, western and north western shores. The combined river flow of the Avon and Stour occupies a shallow tide channel cutting through the sediment flats, but is deflected northward through a very constricted entrance, The Run, before reaching the sea. This deflection, and occlusion of the harbour entrance, is caused by a double spit feature resulting from coastal erosion and longshore drift. With high river flow rates, especially in winter, fresh water can become ponded within the harbour, restricting saline inputs from the sea at the same time. Consequently, most of the harbour is brackish but the degree of salinity reduction varies with tide and the season. THe harbour is attractive scenically, with no industrialised hinterland, and is an important centre on the south coast for recreational water sports. There is little commercial fishing within the estuary apart from netting for salmon, sea trout and eels. The River Avon in particular is noted both for its salmon run and for its coarse fishing. The area receives a lot of visitors, especially holiday makers from nearby Bournemouth and Southbourne, and attractions such as Hengistbury Head to the south of the harbour are under considerable pressure. The harbour is also of ornithological interest, because migrant birds crossing the Channel appear to favour the area as both a landfall and a dispersal point. Stanpit Marsh is a Local Nature Reserve, and most of Christchurch Harbour has been notified as an SSSI. Scientific interest in the past has been centred on the region's geology and physiography, together with its diversity of botanically interesting maritime habitats. The present survey, carried out in June 1987, aimed to collect information on the variety of habitats and communities present subtidally and intertidally in Christchurch Harbour. A total of 32 sites were visited, of which 23 were intertidal and 9 subtidal. At each intertidal site the abundance of flora, epifauna and conspicuous species were recorded in situ. Sediment cores were taken and sieved over a 0.5 mm mesh to obtain quantitative infaunal data. Subtidal areas were sampled using a combination of diving and dredging. Within the harbour, habitat diversity was low and predominantly sedimentary in nature, or sediment influenced. Epifaunal and algal diversity was also low due to the generally reduced salinity regime, and a lack of stable substrata. The most diverse fauna and flora found was on the isolated ironstone boulder outcrops outside the harbour. The macroinfauna was also of low diversity generally and was characterised by very dense populations of a restricted number of species. Within the length of The Run, the fauna changed from a coastal type dominated by Lanice conchilega, to an upper estuarine or brackish type dominated by Streblospio shrubsolii, Corophium volutator and Hediste diversicolor. This fauna type extended across the main open part of the harbour as far west as Wick Hams, after which estuarine species dropped out to be replaced by fresh-water or brackish forms. The scientific and nature conservation importance of the area has been assessed using standard criteria. These have been applied to each of the habitats or community types identified which have then been provisionally graded as being of Local, Regional, National or International importance. No species of particular scientific interest were found during this survey work, but the conservation importance of three invertebrate species recorded by other workers from brackish saltmarsh habitats has been assessed in a similar manner. These were two species of ostracods, Callistocythere murrayi Whittaker and Cytherios stephanidesi Klie, and an amphipod, Gammarus insensibilis Stock. Their gradings are provisional because these groups, and their habitats, have not received wide or intensive study and their distributions still require elucidation. Although the harbour has been assessed mainly as an estuary, it also possesses distinctly lagoonal properties, and the assessment of this interesting coastal inlet may require adjustment as the status of Britains's lagoonal habitats becomes better known.