There is strong evidence indicating that gene flow from transgenic B. napus into weedy wild relatives is inevitable following commercial release. Research should now focus on the transmission, stability, and impact of transgene expression after the initial hybridization event. The present study investigated the transfer of a phosphinothricin-tolerance transgene by inter-specific hybridization between B. rapa and two transgenic B. napus lines. The expression of the transgene was monitored in the F 1 hybrids and in subsequent backcross generations. The transgene was transmitted relatively easily into the F 1 hybrids and retained activity. Large differences in the transmission frequency of the transgene were noted between offspring of the two transgenic lines during backcrossing. The most plausible explanation of these results is that the line showing least transmission during backcrossing contains a transgene integrated into a C-genome chromosome. Approximately 10% of offspring retained the tolerant trait in the BC 3 and BC 4 generations. The implications of these findings for the stable introgression of transgenes carried on one of the chromosomes of the C-genome from B. napus and into B. rapa are briefly discussed.