The risk of release of genetically modified oilseed rape (Brassica napus) was investigated in relation to interspecific gene flow with hoary mustard (Hirschfeldia incana).  Microscopic studies showed polymorphism within the population of hoary mustard for pollen germination on oilseed rape flowers.  The transgenic herbicide-resistant line and a commercial cultivar of oilseed rape were not different for pollen behavior and ovule fertilization.  Pollen tube growth was slow and erratic in interspecific crosses.  Fertilization efficiency of oilseed rape and hoary mustard pollen in interspecific crosses was 15% and 1.3%, respectively, of that in intraspecific crosses.  This unequal efficiency in reciprocal crosses was confirmed by hybrid seed set in pods.  There was no post-zygotic barrier to the development of hybrid embryos in hoary mustard pods.  Up to 26 spontaneous hybrids per male sterile oilseeed rape plant, and one per hoary mustard plant, were obtained in field experiments.  Hybrids were identified by isozyme electrophoresis, morphology, and cytology.  All hybrids were triploid with 26 chromosomes, and had low fertility.  They produced 0.5 seeds per plant after spontaneous backcrossing with hoary mustard.  Some of these descendants were produced from unrelated gametes.  Our results suggest that gene flow is likely to occur, but its actual frequency under crop growing conditions remains to be estimated.