Two opposing interpretations have been made on the depositional origin of the Late Pleistocene Sunnybrook diamicton in the Metro-Toronto region of southern Ontario. The traditional view holds this deposit to be a till, but more recent arguments advocate a glaciolacustrine origin by suspension deposition and ice rafting.The discovery of a low-diversity ostracode fauna consisting of Candona subtriangulata, Candona caudata, and Darwinula stevensoni in the Sunnybrook diamicton suggests a lacustrine origin. Excellent preservation of thin, fragile shells and a mixture of juveniles and adults in sediments of varying texture—clayey silt to silty clay—indicate that there has been little winnowing or selective sorting and that the ostracodes are in situ. Indications are that the Sunnybrook diamicton was deposited in a large, deep, and cold lake.These new palaeontological observations call for a reassessment of the Quaternary stratigraphy of the Ontario Basin and suggest that the Laurentide Ice Sheet in the eastern Great Lakes region was less extensive during the early part of the Wisconsin Glaciation than previously assumed.