The occurence of spontaneous hybridization between Brassica napus (oilseed rape) and Raphanus raphanistrum (wild radish) was investigated under different density conditions in cages and open-field experiments. Hybrids with wild radish as the seed parent were identified by screening for herbicide resistance belonging to rape. Small seed size and intermediate morphology were used to screen for hybrids with rape as the seed parent. Leaf isozyme patterns and flow cytometry provided confirmation of hybrids. Wild radish in an oilseed rape field produced as many as three interspecific hybrids per 100 plants. This is the first report of such a spontaneous event. The frequency of hybrids is expected to range from 0.006 to 0.2% of the total seed produced, at P = 0.05. Male-sterile oilseed rape plants surrounded by wild radish can produce up to 37 hybrids per plant. Seed production of the F1 hybrids and their F2 descendants was up to 0.4% and 2%, respectively, of that of wild radish. Gene escape from transgenic oilseed rape to wild related species is discussed.