The Don Formation comprises 8–9 m of fossiliferous stratified sand and clay, lying between glacial deposits of Wisconsinan and presumed Illinioan age. It has yielded a diverse fossil flora and fauna indicating a climate as warm as or warmer than present and is considered to be of last, or Sangamonian, interglacial age. About 2 t of sediment were processed from the type section of the Don Formation at Toronto, Ontario, and yielded approximately 18 000 identifiable mollusc shells, representing 45 taxa. Previously unpublished taxa include 11 freshwater gastropod species, 12 pelecypod species (Sphaeriidae), and 7 terrestrial gastropod species. The molluscs are mainly concentrated in the lower two-thirds of the formation and are dominated by Valvata perdepressa, with abundant Valvata sincera, Probythinella lacustris, Amnicola limosa, Amnicola walkeri, Pleurocera acuta, Elimia livescens, Pisidium casertanum, Pisidium compressum, Pisidium fallax, and Sphaerium striatinum. All the mollusc species are modern inhabitants of the Great Lakes region. Their known ecological tolerances suggest deposition in relatively shallow waters of a large temperate lake having substantial fluvial input nearby. Stratigraphic variations in the relative abundance of different aquatic taxa may reflect variations in water depth or fluvial activity. Stable isotope data from several mollusc species and a single wood specimen, together with taxonomic paleoenvironmental interpretation, are consistent with deposition during the climatic optimum of the Sangamonian interglaciation. Environmental conditions and faunas are similar to those of the Nipissing phase (Hypsithermal) of the eastern Great Lakes.