This paper describes the non-marine Molluscan faunas of riverine interglacial deposits from Sugworth, near Oxford. The faunal assemblages studied indicate the former presence of a large, well oxygenated river, a proto-Thames, fringed with marsh and flowing through a fully wooded, temperate interglacial landscape. Freshwater, riverine taxa predominate, with variations in the molluscan assemblages principally reflecting local changes associated with the river shifting its course. Derivation, transportation and winnowing are thought to be important in the formation of the assemblages. The balance of available evidence indicates that the deposits should be assigned to Zone CrIIIb of the Cromerian interglacial. Declines in both the numerical abundance of specimens per sample and the species numbers per sample, noted through the profile, may have an underlying climatic cause.