The process of introgression between a transgenic crop modified for better agronomic characters and a wild relative could potentially lead to increased weediness and adaptation to the environment of the wild species. However, the formation of hybrid and hybrid progeny could be associated with functional imbalance and low fitness, which reduces the risk of gene escape and establishment of the wild species in the field. Risk assessment studies of transgenic crops have recently brought evidence of a low spontaneous hybridization frequency of Brassica napus with Raphanus raphanistrum. The fate of the first generation hybrids is crucial to determine the initial rate of spread of transgenes. In the first part of our work, we compare the fitness components of parents and F1 hybrids at the first step of the life cycle. The F1 hybrids show a lower seedling emergence, a significant delay of emergence, and a lower survival than for both parents. Rosette diameter and dry matter of hybrid plants are reduced relative to those of both parents, especially when they grow in the field and under competition conditions. Experiment in controlled growth conditions are unable to predict the differences observed in the field. In the second part of our work, we compare the fitness components of parents and different types of backcross in the sixth generation of hybrids. The backcross with oilseed rape cytoplasm has a fitness value 100 times lower than that of the backcross with wild radish cytoplasm. The herbicide resistant BC6 has similar growth as the susceptible counterpart, but final male and female fitness values are two times lower. In turn, susceptible BC6 exhibit similar fitness as the control wild radishes. The consequence on fitness of the chromosome location of the transgene conferring resistance and the relevance of these results to the impact of gene flow on the environment are discussed.