Drug use among transnational Mexican migrants, predominantly solo males, is drawing public concern in many agricultural communities across the country. The current knowledge attributes the problem primarily to situational factors associated with living and working in US farming regions, but fails to consider other community factors in the United States (i.e., drug availability and the presence of a drug subculture in local communities) and Mexico (i.e., drug availability and the presence of a drug subculture) as well as individual factors (i.e., background characteristics and predisposing factors). The roles of these community and individual factors are unclear, as are the relationships between these factors and their specific connections to drug use. We propose a bi-national social ecology model of drug use comprised of community, individual, and other factors in the United States and Mexico to understand the manner in which these factors contribute to drug use among transnational migrants. This model has not been elaborated or verified because few studies have been conducted in both the U.S. agricultural work sites and in migrant home communities in rural Mexico, and because of the lack of coordinated bi-national drug use studies that consider factors in both countries. In order to substantiate and elaborate on the key factors in the social ecology model, the contributing factors in both the United States and Mexico will be explored through four progressive, ethnographic field studies conducted over a three-year period. The field studies will be conducted in Southern Chester County, a major mushroom producing region of southeastern Pennsylvania, and in three municipalities in Guanajuato, Mexico, location of the home communities of the Mexican migrants working in Pennsylvania. This approach is essential to gaining access to the migrant population and to describing the complexity of factors contributing to drug use among the migrants. In each of the four field studies, the researchers will reside on site and gather qualitative data using observations, ethnographic interviews, informal interviews, focus groups, social network analysis, and genealogies. The findings will contribute to a comprehensive understanding of drug use among migrant farmworkers and to the design of bi-national substance abuse intervention and prevention programs as called for in public health policies. These programs will address the causes of substance abuse on both sides of the U.S.-Mexico border.