There seems to be little doubt that major population shifts occurred in Northern Ontario and Manitoba following the Iroquois wars of the seventeenth century and the advent of the fur trade. These movements have tended to blur pre-contact ethnic boundaries. Although recent archaeological reports have provided much needed data, differing interpretations of that data have only further obscured the issue. For instance, while some scholars have argued that Blackduck Focus sites in western Northern Ontario and northern Minnesota were occupied by Algonkian-speakers (either Ojibwa or Creej, others have interpreted the materials as having an Assiniboin authorship. In support of the latter view, we analyze both the archaeological and early ethnohistorical data. From these sources we conclude that, at contact, the Ojibwa extended no further west than Michipicoten Bay, while the Cree occupied most of Northern Ontario except for a strip of about 50 mi north and south of the present International border west of Lake Superior which was Assiniboin territory. Proper determination of ethnic boundaries is of theoretical importance to studies of cultural ecology and social organization.