The molluscan fauna of the interglacial channel deposits at Little Oakley, Essex, U.K., is described. Detailed faunal analyses are presented from three boreholes located at different sites across the palaeochannel. Large-bulk samples (ca. 1.5 t), obtained from mechanically excavated pits at one of these sites, have yielded a few additional species and enabled the resolution of several taxonomic problems that result from the fragmentary nature of some of the borehole specimens. The fauna as a whole is fully temperate throughout and indicates the presence of a large, well-oxygenated river, thought to be the ancestral Thames. Mollusca from fringing marsh habitats are also present, together with xerophilous species from dry calcareous grassland. Woodland species are virtually absent, suggesting that the river here had a wide, open floodplain. The fauna indicates a Cromerian age and includes the extinct prosobranch Tanousia (= Nematurella auctt.), which is unknown anywhere in northwest Europe after this interglacial stage. Other noteworthy species include Sphaerium solidum (only British Pleistocene record), Belgrandia marginate, Helicella itala and cf, Cernuella virgata (earliest British Pleistocene records), Trochoidea geyeri and Truncatellina cylindrica (only British Cromerian records). There are significant differences between the faunas at each of the sampling sites, which are thought to reflect mid-channel and marginal facies. This interpretation is in complete accord with the sedimentology and gives strong support for the local palaeogeographical reconstructions. There are also some biostratigraphical changes: Tanousia declines upwards and is totally absent in the upper levels of one borehole that is thought to extend later into the interglacial. The significance of this disappearance as caused by either regional (climatic) or local factors is unclear.