Brassica napus is one of the crops at the forefront of biotechnological development. The procedures used to produce transgenic varieties are all prone to generating plants with multiple transgene copies. There has been considereable interest in the behaviour of such multi-copy lines because gene dosage rarely appears to correlate simply with gene expression. Here we report the analysis of a population of 107 progeny from a B. napus transformant containing multiple copies of a GUS marker gene construct. A total of 12 GUS sequence copies were identified including one that was non-functional. The expression of GUS increased with increasing copy number but this increase only made up a small proportion of the total variation between lines. There was no evidence of interaction between the various GUS copies and they appeared to segregate independently. The variation between progeny lines indicated that the number of gene copies was not a good guide to the expression of the gene product and hence that the expression of the gene in progeny from a multiple-copy parent could not be predicted. The importance of these findings in relation to plant breeding and the risk assessment process is discussed.