The discovery of transgenic products in maize (Zea mays) landraces planted by small-scale Mexican farmers (Quist and Chapela, 2001, 2002; Christou, 2002; Editorial Note, 2002) raised questions about how the commercial introduction of transgenic maize varieties might affect the traditional agricultural systems of small-scale farmers. A key concern is whether their introduction will have a deleterious effect on the diversity of maize landraces that these farmers maintain. Mexican agriculture, including maize production, has a bimodal structure (Bailey and Roberts, 1983; Nadal, 2000). On the one hand, a large number of small-scale farmers in rain-fed areas grow maize mainly for domestic consumption, though they may occasionally sell some surplus. On the other hand, a relatively small number of commercially oriented farmers practice large-scale maize production, mainly in irrigated areas, and their objectives and technological needs resemble those of their counter-parts in the industrialized world. These large-scale farmers are the logical market for commercial transgenic maize varieties. Although small-scale farmers, who rarely purchase commercial seed, are an unlikely market for transgenic varieties, the introduction of transgenic maize varieties may nevertheless have important consequences for them. In this paper, we identify some of those consequences and explore their implications. Specifically, we examine how transgenes may diffuse in traditional agricultural systems, describe the mechanisms that encourage their diffusion, and discuss the implications for the loss of maize diversity and for biosafety risk assessment. Finally, we indicate how the issues surrounding the diffusion of transgenic varieties in Mexico's traditional systems may prove relevant in other countries and crop species. An important concern in assessing the risk of growing a genetically modified crop in its center of domestication (i.e. where its wild relatives are present) is gene flow between the transgenic crop and its wild relatives. Even though data on this subject are limited, the potential impact of such gene flow has been under discussion for some time (Serratos et al., 1997; Blancas et al., 2002; Gepts and Papa, 2003). In this paper, we will focus on the less-explored issue of maize-to-maize gene flow, which plays an important role in the evolution of maize populations in Mexico (Louette et al., 1997; Perales et al., 2003a, 2003b) and other countries, such as Burkina Faso (Sanou et al., 1997).