Hybridization of Brassica napus L. (oilseed rape) and Raphanus raphanistrum L. (wild radish) has been demonstrated, and may be the first step towards introgression of transgenes in this wild relative. If wild radish were to display a new adaptive advantage by expressing the transgene, this could modify the ecological balance of species within the agro-ecosystem. To determine if transgenes remained stable in the hybrid, the frequency of herbicide resistance was studied over four advanced generations of hybrid progeny (G8 to G11) that were subjected to herbicide selection pressure. It is expected that hemizygous resistant plants containing an herbicide resistance transgene back-crossed to wild radish would have 50% resistant progeny. In each of the G8 to G11 generations, only 18% of the progeny from resistant plants were resistant. The chromosome complement of herbicide-susceptible progenies, analyzed at G9, was not different from that of wild populations of wild radish. Herbicide-resistant G9 progeny showed higher chromosome instability, and one third of the progeny contained a supernumerary chromosome. These results suggest that in the presence of herbicide selection pressure, the transgene for herbicide resistance would be maintained despite a lack of stabilized introgression. In the absence of selection, the frequency of resistance in the population is expected to decline.