The role of crop-to-weed gene flow is often controverrsial and overlooked. As a consequence, the likelihood of spontaneous crop-to-weed hybridization in most crop-weed systems is generally unknown. The lack of data relating to the formation of crop-weed hybrids has particular contemporary significance when considering the wide scale commercial release of transgenic crop plants and thr potential for escape of engineered genes via crop-to-weed hybridization. We created an experimental system whereby we could examine the incidence and rate of spontaneous crop-to-weed hybridization between Sorghum bicolor and S. halepense, jonsongrass. An isozyme marker was used to identify hybrid plants ghrough progeny testing. Incidence and rate of hybridization were highly variable with respect to weed distance from the crop, location of the study size, and year the study was performed. Crop-weed hybrids were drtected at distances of 0.5-100 m from the crop. Interspecific hybridization can and does occur in this system at a substantial and measurable rate. Transgenes introduced into crop sorghum can be expected to have the opportunity to escape cultivation through interspecific hybrdization with johnsongrass. Trais that prove to be beneficial to weeds possessing them can be expected to persist and spread. This is an issue that needs to be addressed when developing biosafety guidelines for the commercial release of transgenic sorghums.