A sand pit at Leamington, Ontario, in southeastern Essex County in the southernmost part of Canada, has yielded many taxa of fossil animals and plants that are dated over 13 000 BP. The fossils are of both terrestrial and freshwater origin, and comprise a surprisingly diverse but coherent assemblage of molluscs, ostracodes, Chironomidae, Coleoptera, Turbellaria, Trichoptera, and a few fish bone fragments. This is the oldest known postglacial occurrence in Ontario for all animal groups. Paleoenvironments indicated by the taxa range from boreal forest to tundra; taxa are an in situ assemblage with little transport or reworking. Although this site has yielded the richest fossil record of this age yet found in Ontario, previous finds at a few sites on the north edge of the eastern Erie basin and in the southern Huron basin indicate plants were well established in southwestern Ontario by the time of the Mackinaw Phase interstadial when lowered lake levels likely facilitated their arrival from land areas to the south. This discovery greatly improves the prospects of finding fossils of this age even though the known record is still extremely limited. The site represents fortuitous preservation and discovery, and amply demonstrates that glacial lakes of this time, when much of the last ice sheet was still in existence, were far from barren of life and that the migration of biota into the area was quite rapid during ice retreat.