Prior to the commercialization of Roundup Ready^® alfalfa (Medicago sativa L.), an ecological risk assessment was conducted. As part of the risk assessment, the potential for and consequences of the transfer of the Roundup Ready trait to feral alfalfa were evaluated. Biogeographic data were used to assess the potential for gene flow from cultivated alfalfa to feral alfalfa. A biogeographic survey was conducted in 2001 and 2002 to provide information on feral alfalfa populations within five major U.S. alfalfa production states. A total of 940 roadside sites were surveyed among 47 counties in California, Idaho, Pennsylvania, South Dakota, and Wisconsin. At approximately 22% of the sites, feral populations were located within 2000 m of cultivated alfalfa. On average, feral populations occupied < 3% of the area surveyed (500 m² per site). The proximity of feral populations to cultivated alfalfa suggests that gene flow will occur between these populations. Gene flow in seed production regions, however, may be limited by the management practices used by seed producers to control feral populations and to ensure varietal genetic purity. In forage production systems, pollen flow from cultivated alfalfa is minimized by continual harvest of the forage at early bloom throughout the growing season. The consequences of gene flow from cultivated Roundup Ready alfalfa to feral alfalfa, in terms of increased pest potential, are low because (1) phenotypic evaluations concluded that the introduction of the Roundup Ready trait does not increase the fitness of alfalfa, (2) feral populations are not typically controlled using herbicides, and (3) where controlled, glyphosate is not the only herbicide used as other more effective herbicides are available.