A Plant Heat-Zone Map of Mexico has been prepared, based on the average number of days in a year with highs over 30C. This is similar to the Plant Heat-Zone Map of the United States, copyright by the American Horticultural Society. The Mexico map has thirteen zones instead of the twelve zones of the United States, because of Mexico’s higher temperature regime. Heat-Zone maps have proven useful for classifying plants by their resistance to summer heat conditions. The Mexico map was prepared from the daily temperature records in Eric II, the CD-ROM published by the National Water Commission of Mexico, with certain corrections furnished by Eric’s author. In addition to its normal use as a paper map for horticulturists, it will be used in electronic form for predicting potential distributions of wild plants and ornamentals. It may also be useful for studies of the natural or potential distributions of insects and other organisms.