A Drosophila transposable element, the P element, containing the bacterial gene encoding beta-galactosidase is widely used to search for tissue-specific enhancers. This lac-Z-containing P element (P-lacZ ry+) can be moved around the genome by a number of techniques. When it comes to lie close to a tissue-specific enhancer, blue staining results in particular tissues of the fly. Many different patterns of expression have already been obtained with the long-term aim of cloning the nearby genes that these enhancers normally regulate. Whilst analysing a set of flies containing these P-lacZ inserts for sex-specific expression in the adult, a preference was noticed for insertion into regions of DNA generating expression of beta-galactosidase in the male gonad. Since the transposition events generating these flies occurs in the male germ-line, it seemed possible that there was preferential insertion of the element into DNA which was being transcribed. To test this, transpositions were generated of the same P-lacZ ry+ in both the male and female germ-lines. The results are compatible with the above hypothesis. This finding has important implications for the type of enhancers likely to be found by this method, and may also be relevant to those using P elements as mutagens in Drosophila and for the study of the mechanism of P transposition.