As the development of transgenic Brassica napus L. strains moves into field trials in open agricultural settings, there is the need for pollen flow to be controlled. Current Canadian Government regulations require either large isolation zones (200 m) or 10 m wide borders of synchronously flowering non-transgenic B. napus. Border areas of 15 to 30 m wide were planted around a 60 m x 30 m central plot of bromoxynil herbicide resistant transgenic B. napus strains for four field trials conducted in Carman and Winnipeg, Manitoba in 1994 and 1995. Seed samples were taken from the border area at 0, 2.5, 5, 10, and 15 m for four cardinal directions and additionally at 20, 25, and 30 m for two cardinal directions. These seed samples were planted in the field in 1995 and 1996 and screened for the presence of bromoxynil resistant plants (i.e. plants which were the result of an outcrossing event). For the four trials combined, outcrossing rates varied with by year, direction and distance. Outcrossing rates averaged about 0.70% at 0 m and declined to 0.02% at 30 m. The introduction of pollinators (leafcutter bees at the Carman site in 1994 and 1995) did not appear to significantly influence outcrossing rates. The bromoxynil resistant plants were screened for heterozygosity of the bromoxynil resistance trait to ensure that they were the result of an outcrossing event and not the result of contamination by a pure breeding (i.e., homozygous) bromoxynil resistant plant. Two hundred and eighty putative F$\sb1$ plants were screened for heterozygosity of the bromoxynil resistance trait. Over 93% of the putative F$\sb1$ plants screened were found to be heterozygous, i.e. true outcross plants.