The transfer of engineered genes (transgenes) from crops to natural populations will depend first on mating between the crop and related weeds and then upon the relative fitness of the weed-crop hybrid. While weed-crop hybridization is known to occur readily under agricultural conditions, almost nothing is known of the fitness of the hybrids produced. Therefore, we measured the relative fitness of weedy radishes and their sibling weed-crop hybrids under field conditions. Specifically, we compared germination success, time to first flowering, fruit production, seed production, and frequency of transmission of the crop allele to seed progeny. Hybrids showed significantly greater fruit and seed production, and equaled weeds in all other measured characters. Thus, in this experiment, the fitness of hybrids exceeded that of their wild siblings. These results suggest that, in at least this system, neutral or advantageous transgenes introduced into natural populations will tend to persist.