Mollusks were studied from six sites in Lake Algonquin deposits (12,000-10,000 yr B.P.), five transitional (Lake Stanley low stage; 10,000 – 6000 yr B.P.), and six Nipissing stage sites (6000-4000 yr B.P.) east of Lake Huron in southwestern Ontario. The sites represent a variety of near-shore, lagoonal, estuarine, and fluvial environments. Eighteen species were limited to occurrences in Algonquin stage deposits; 8 were found only in the transitional age sites; and 14 species were restricted to Nipissing stage localities. With the possible exception of Goniobasis livescens, which occurred at five of the six Nipissing stage sites, the remaining stratigraphically limited species were usually restricted to one or two localities and probably cannot be used as zone fossils. Some cold-tolerant species (e.g., Anodonta grandis simpsoniana) were very early migrants into the study area, while others arrived later, apparently from eastern, southern, and western sources. Mollusks proved useful in paleoenvironmental reconstructions and to a lesser extent in biostratigraphic zonation.