This paper reports new data on stable carbon and nitrogen isotopes obtained from human skeletal remains found at six prehistoric sites dating between A.D. 400 and 1500 in southern Ontario. Analyses examine more closely the timing and intensity of maize adoption and the importance of animal protein in the diet, adding to earlier work in the region by the same authors (Schwarcz et al. 1985). As a result of changes in preferred methods of extracting bone collagen, a comparison of extraction methods is presented. Results indicate a gradual increase in the importance of maize in the diet over a period of approximately 600 years, from A.D. 650 to 1250, and little change in nitrogen isotope values during the same period. The results are considered within the larger temporal and geographical framework of eastern North America, drawing on stable isotope results from the published literature. Both paleo-botanical and isotope data indicate marked differences in the timing and intensity of maize utilization in different regions of northeastern North America. Nitrogen isotope values decrease after around A.D. 1350, suggesting a decrease in animal protein in the diet. Stable isotope data provide one source of evidence for changes in human subsistence patterns and their interpretation relies on complementary data from sources such as the analysis of faunal and botanical remains, settlement patterns, and material culture.