Gene flow resulting from cross pollination becomes an issue when transgenic crops are involved and the genetic modification carries a trait of ecological importance. As crop fields are often separated by a barren gap, such as an intervening roadway or unplanted area, I measured cross contamination between two herbicide-resistant transgenic fields (canola, Brassica napus) across a gap of up to 12 m. I focused on pollen exchange from the field border up to 7 m inside each field over two seasons. In the absence of a gap, I found that gene dispersal diminished rapidly with distance, with more than 40% of transgenic progeny found within the first meter from the edge of the adjacent crop. Cross contamination between fields declined more rapidly when there were intervening plants, however. Plants separated from the transgenic source by a gap of 3-4 m, yielded the same level of transgenic progeny as those separated by 1 m of crop. Both insects and wind pollinate canola, and so the explanation for my observations could involve the influence of gaps on wind patterns or on the behaviour of pollinators. The gap effect does not seem to depend only upon the variation in the density of neighbours that surrounds those plants at the crop edge versus those in the crop matrix. On the basis of this study, it is recommended that economic profit would be maximised by removing field borders after flowering rather than by leaving a surrounding gap, which would need to occupy up to threefold as much field surface to achieve the same level of containment.