Most native plants of Ontario use the C₃ photosynthetic mechanism and therefore have relatively low ¹³C/¹²C ratios; maize is a C₄ plant and is relatively enriched in ¹³C. The proportion of maize in the diet of a human can therefore be estimated from the ¹³C-content of collagen of fossil bone. We have used this method to follow the increase in consumption of maize in ancient native populations over the interval from AD 400 to 1650, and to quantify the amount of maize consumed. Maize consumption rose gradually to a maximum of 50% of the diet by AD 1400. These data agree with archaeologically derived data for this region, and with previous isotope studies in the northeastern U.S.A. In some of the latter, however, the maximum percentage of maize consumed has been overestimated because maize was assumed to have a δ¹³C value of −12·5‰; our studies of archaeological specimens show the correct value to be closer to −9‰. The introduction of beans into the native diet about AD 1100, should have caused a decrease in the ¹⁵N content of human bone collagen because legumes are deficient in this isotope, with respect to meat and fish. However, we observe no significant change in the ¹⁵N/¹⁴N ratio of human bone collagen over the period from 2300 BC to AD 1640. We conclude that meat and fish remained the main sources of protein even after the advent of agriculture into this region.