Transgenic crop varieties are a rapidly expanding and controversial technology. Their effects on biological and cultural diversity are a key issue in an often polarized debate. Here we provide answers to questions about one important example, that of transgenic maize in Mexico. In situ maize diversity in Mexico is present in traditional varieties in farmers' fields, and in wild and weedy relatives of maize. It is likely that transgenes are present in farmers' local maize varieties, but it is unknown whether they have introgressed. Socioeconomic changes, including migration, trade liberalization, and reduced support for Mexican farmers, may also affect maize diversity. Diversity may increase, decrease, or remain the same, but whether this is viewed as good or bad will depend on subjective values.