A widely acknowledged risk associated with transgenic crops is the possibility that hybridization with wild relatives will cause fitness-related transgenes to persist in wild populations. If wild plants acquire transgenes coding for resistance to herbivory, disease, environmental stress, and/or commonly used herbicides, they could become more abundant in their natural habitats or invade previously unsuitable habitat. In addition, wild populations containing transgenes that provide resistance to herbivores or disease may have additional effects on natural insect and pathogen populations. However, little is known about whether these concerns are justified. For example, many weedy species are affected by herbivores, yet the impact of insect damage on population densities and invasiveness has rarely been examined. In order to determine if a transgene poses a risk to wild populations, or to the species with which the wild plant interacts, three questions must be addressed. These are: 1) Are there genetic or geographic barriers to the escape of the transgene from the crop into wild populations? 2) Is the transgene expected to increase in frequency in wild populations? And 3) What are the ecological consequences of escape? We are investigating these questions in the cultivated/wild sunflower system. In answer to the first two questions we have found: few barriers to gene flow between the crop and wild population, and decreased lepidopteran herbivory and increased seed production in wild plants containing a Bt-toxin gene specific to lepidopterans. In answer to the third question, preliminary analyses suggest that increased seed production in individual plants (caused by, for example, a Bt gene) will lead to an increase in the size of wild populations. We are currently doing experimental and modeling work to determine if larger local populations will lead to larger or more persistent metapopulations.