Three main types of Hebridean freshwater lochs occur in the Loch Druidibeg National Nature Reserve; eutrophic calcareous machair lochs near the Atlantic coast, mesotrophic lochs on the edge of the moor and oligotrophic lochs on the peatlands. Chloride/calcium concentrations, highest in lochs near the sea, fall away rapidly inland and are lowest in the moor lochs. The fauna of eutrophic and mesotrophic lochs includes a few euryhaline brackish-water species Potamopyrgus jenkinsi, Diaptomus wierzejskii, Neomysis integer and Gammarus duebeni. G. duebeni alone penetrates the oligotrophic lochs. Freshwater sponges are widespread but are absent from the machair lochs and there are no triclad planarians. The fauna of fast and slow stream segments is described. Loch Bee, a man-made lagoon, unlike the open brackish lochs of North Uist, is an unstable habitat and its fauna has changed considerably since it was investigated in 1933. The fauna consists mainly of brackish-water species and a few stenohaline marine species which are confined to its seaward ends. The sequence of faunistic change in the Howmore estuary, Lochs Roag and Fada, a Site of Special Scientific Interest (SSSI), is described; the brackish-water fauna is reduced by the volume of fresh water passing through the system. A brief account is given of the unusual dominance of Hyalinella punctata and Potamopyrgus jenkinsi in Loch Ardvule (SSSI), South Uist.