Hybridization between the herbicide-resistant transgenic crop Brassica napus L. (canola) and its weedy relative Brassica rapa L. (bird rape) has been documented in Quebec.  Our goal was to evaluate the actual hybridization potential based on range overlap and actual in situ hybridization rates.  This was done by mapping B. napus canola fields, comparing them with the sampling locations of B. rapa herbarium specimens from Quebec, gathering information on the presence of B. rapa in certified canola seed production fields, and surveying for B. rapa populations located in, or close to B. napus field margins.  Progeny from these populations were screened for herbicide resistance (HR) and for the presence of the HR transgene.  Two fields were also selected to evaluate B. rapa density effects on hybridization rates.  Significant sympatry was observed in several areas of the province; hybridization occurred in all eight populations (1.1% to 17.5% hybrid seed) located in field margins and in one (1.1%) out of three populations located less than 10 m from a B. napus field.  Hybridization rates decreased exponentially as B. rapa density increased, but interplant rates (0% to 68%) were highly variable.  Environmental problems could be generated by the release of B. napus crops with traits conferring fitness benefits in nonmanaged areas.