In northwest Mexico, along the Sierra Madre Occidental and the adjacent slopes and plains, maize forms the center of a series of cultural traits illustrative of the interrelationships of man and plants. Here more than 20 racial types have been collected. This diversity is related to the variation in climate and soil conditions, to human migrations from the Mesoamerican cultural center to the Greater Southwest area, and to the differential selection by the ethnic groups in the region. The agricultural practices of the farmers are reviewed to illustrate how different aspects of environment, economic needs, food preference, manner of utilization, and ceremonial concepts constitute continuing forces of selection and motivation for introduction of varieties from other areas. It is suggested that color of the grain is utilized as an indicator of physiological characteristics. A phenotypic analysis of more than 600 collections from the area indicates the occurrence of constant intercrossing among maize populations.