Introgression between genetically modified (GM) crops and wild relatives is considered to potentially modify the genetic background of the wild species. The emergence of volunteer-like feral populations through backcross of hybrids to the crop is also a concern. The progeny of spontaneous hybrids between mutant herbicide-resistant oilseed rape (Brassica napus) and wild B. juncea was obtained. Parents, F2 and BC1 to B. napus were planted together in the field so as to study their performance. The chromosome number of BC1 followed a Normal distribution. Mendelian ratio of the herbicide-resistance gene was found. The F2 produced less seeds than B. napus, and BC1 had intermediate production. Herbicide-resistant BC1 were not different of their susceptible counterparts for plant weight, seed weight and seed number, but most of them exhibited B. napus morphology and larger flowers than the susceptible BC1. They displayed an important genetic variability allowing further adaptation and propagation of the herbicide-resistance gene. Pollen flow to susceptible plants within the mixed stand was observed. As a consequence, the resistant BC1 produced with B. napus pollen could frequently occur and easily establish as a false feral crop population within fields and along roadsides.