The time of introduction of maize, a C₄ plant (dicarboxylic acid path of photosynthesis), in North American diets was estimated by determining the ₁₃C content, relative to the PDB standard, in adult human skeletal collagen, as compared to C₃ (Calvin path of photosynthesis) plants, taking into account the enrichment factor in collagen. Thus, there was no measureable C₄ component in the diet of lower Illinois valley dwellers in the Archaic, Middle Woodland, or even the beginning of the Late Woodland periods (about 8000 to 600 B. C.), but in the end of the Late Woodland period, (about 1000 A.D.), clear evidence of maize cultivation was observed in a C₄ diet component of 24%, after which time there was a rapid increase to about 50% by about 1200 A.D. In Ohio, a wide gap was observed in isotopic values between hunter-gatherers and fully agricultural people, and in the latter there was also a sex difference seen as a lower C₄% in the diet of females. -- AATA