Carbon and nitrogen isotope ratios in collagen from bones of individuals who lived in the Tehuacan Valley during the period 8000–1000 years B.P. have been interpreted as indicating earlier use of maize and more utilization of legumes as food sources than is suggested by the occurrence of the remains of these plants in the coprolites and debris excavated along with the bones. Reassessment of the assumptions made in interpreting the bone collagen isotope ratios reduces some but not all of the discrepancy between the isotopic and archaeological reconstructions of diet. The original archaeological reconstruction relied entirely on remains from cave sites, thus introducing seasonal and locational biases into the dietary reconstruction. Using the bone collagen isotope ratios as a guide, we re-interpreted the archaeological data to produce a more complete picture of temporal changes in the overall diet. We suggest that heavy dependence on grains began in the Coxcatlan phase and then may have remained unchanged for the next 5,500–6,500 years. These conclusions, which are based on a relatively small isotopic data base, need to be verified by an extensive program of isotopic analysis of the floral, faunal, and human remains from Tehuacan.