Zea mays subsp. mexicana, the Chalco teosinte, was recently shown to be widespread in the Valley of Toluca. It is distributed in a larger area than the Chalco populations themselves. Also, it grows at higher altitudes. Its presence is supposed to be the result of introduction. Several surveys of teosinte and weeds by competent biologists in the 1960's failed to find the populations. Herbarium specimens only exist for the last 10 years. - In 1995, we systematically interviewed farmers about their recollections of the time of the introduction. The results show that teosinte has been present for at least 40 years, so they were missed in the 60s. Perhaps the populations were still small, and restricted to a not very accessible area on the former lake bottom in the southeast of the Valley. Teosinte is still in expansion. Comparison of the interviews with population density data show that the likely area of early introduction coincides with the highest population densities today. - It is probable that teosinte was introduced inadvertently, as numerous folk names are applied to the plant in the Valley, many of which appear to be newly minted. The widespread common name of "acece" in the Valley of Chalco was not found, nor the names derived from the roadrunner bird, huiscatote, which are associated with the Balsas teosinte growing a few kilometers further south and west.

Key words: ethnobotany, invasion, Mexico, teosinte, Toluca valley, Zea mays ssp. mexicana