The stable isotopic (δ18O δ13C) composition of freshwater microcrustaceans (ostracodes) is presented for a long (100 m) sequence off late Pleistocene glaciolacustrine sediments at Toronto, Canada. This section may span a substantial part of the last (Wisconsin) glacialcycle from the close of the last (Sangamon) interglacial (oxygen isotope stage 5) to the glacial maximum after 25 ka (O-isotope stage 2). A well-defined secular trend in both δ18O and δ13C for two benthic ostracode species Candona subtriangulata and Candona caudata contained in these early and middle Wisconsin sediments records the increasing contribution of glacial meltwater to the basin; isotopically lightest ostracodes (δ18O = 17‰) are toward the top of the glaciolacustrine stratigraphy immediately below the erosive base of a late Wisconsin till. In the last stages of the lake, δ18O of the lake water ranged between -17‰ and -21‰, implying that the lake consisted of between 35% and 55% glacial meltwater. Consistent differences in isotope values for the two ostracode species suggest an isotopically stratified lake and confirm modern ecological data that the two species have different depth preferences. This study provides important information regarding the isotopic composition of meltwaters from the Laurentide ice sheet and may ultimately help constrain the relation between ice volume over the North American continent and δ18O of sea water. This study confirms other glacial geologic data from mid-continent North America which indicate that the maximum regional extent of the Laurentide ice sheet was attained after 25 ka within oxygen isotope stage 2.