The Uto-Aztecan language family figures prominently in research on early agriculture in western North America. A central issue is the role that the members of the Proto-Uto-Aztecan speech community might have played in the diffusion of maize agriculture from Mesoamerica to the southwestern United States. Key to addressing this issue is determining whether an agricultural lexicon can be reconstructed for Proto-Uto-Aztecan, but despite several comparative studies of the agricultural lexica of the Uto-Aztecan languages, consensus remains elusive. A detailed reanalysis of these lexica indicates that an agriculture-related vocabulary can be reconstructed only for Proto-Southern Uto-Aztecan, supporting the conclusion that maize agriculture entered the Uto-Aztecan world after the division of the Proto-Uto-Aztecan speech community into southern and northern branches. Additional lexical and biogeographical data suggest that the Proto-Southern Uto-Aztecan speech community was located near the modern Arizona-Sonora border when its members began cultivating maize, a development that may have occurred around four thousand years ago, when the earliest evidence of maize agriculture appears in the archaeological record of the North American Southwest.