Concerns have been raised about the possibility of sexual transfer of herbicide resistance genes from transgenic crops towards weedy relatives. The average rate of spontaneous hybridization between Hirschfeldia incana (L.) Lagrèze-Fossat and oilseed rape (Brassica napus L.) was 0.6 hybrids per plant over 3 years of field experiments using herbicide-resistant oilseed rape as a pollen donor. Self-incompatibility was shown to be the mating system of most individuals within a population of H. incana, although some plants had some ability to self-fertilize, which could mitigate gene flow. Back-crossing interspecific hybrids to H. incana over five generations showed that introgression was not successful in our experiment.