The expression of several neighbouring genes on plasmid pAD1 that are necessary for conjugation depend on induction with sex pheromone cAD1. Analyses of transcripts by Northern blot hybridization demonstrated that the genes sea1 (encoding surface exclusion protein) and asa1 (encoding aggregation substance) are transcribed independently. Both genes are organized in different operons together with neighbouring open reading frames of unknown function. Several transcripts could be identified for sea1 and asa1. Their transcriptional start sites were determined by primer extension experiments, confirming the results of the Northern blot experiments. We also could identify sea1- and iad- (encoding an inhibitory peptide counteracting sex pheromone cAD1) specific transcripts which are expressed constitutively, but to a lower extent relative to induced conditions. In addition, we localized the asp1 gene coding for aggregation substance of sex pheromone plasmid pPD1 and determined its DNA sequence, which was found to be highly homologous to asa1 (aggregation substance gene of pAD1) and prgB (aggregation substance gene of pCF10). The structural genes were found to be organized more or less identically on the three sex-pheromone plasmids pAD1, pCF10, and pPD1, and to be highly conserved. Regions supposed to be of crucial importance for regulatory functions, however, were found to differ. We also could identify some conserved DNA motifs which might be potential target sites for transcriptional regulators. In combination these data allowed us to formulate a model for the regulation of sex-pheromone-inducible genes of plasmid pAD1. Its main statement is that only in the presence of cAD1 can the gene traE1 be transcribed. The positive regulatory factor TraE1 then can trigger expression of the structural genes sea1 and asa1.