The virulence of the opportunistic pathogen Pseudo-monas aeruginosa is largely dependent upon the extracellular production of a number of secreted proteins with toxic or degradative activities. The synthesis of several exoenzymes is controlled in a cell-density-dependent manner by two interlinked quorum-sensing systems. Their secretion across the outer membrane occurs through the Xcp translocation machinery. The xcp locus located at 40 min on the chromosome consists of two divergently transcribed operons, namely xcpPQ and xcpR to xcpZ. In this study, transcriptional fusions were constructed between the xcpP and xcpR genes and the lacZ reporter. Transcriptional activation of the xcpP and xcpR genes in P. aeruginosa is growth-phase dependent and the lasR–lasI auto-induction system is required for this control. In the heterologous host Escherichia coli, the lasR gene product, together with its cognate autoinducer N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL), activates both the xcpP–lacZ and the xcpR–lacZ gene fusion. The second P. aeruginosa quorum-sensing modulon rhlR–rhlI (vsmR–vsmI) is also involved in the control of the xcp genes. Expression of the lacZ fusions is strongly reduced in PANO67, a pleiotropic mutant defective in the production of N-acyl-homoser-ine lactones responsible for the activation of RhlR. Furthermore, introduction of the lasR mutation in PANO67 results in additional diminution of xcpR transcription , indicating that the two systems can regulate their target genes independently. These data demonstrate that expression of the xcp secretion system depends on a complex regulatory network involving cell–cell signalling which controls production and secretion of virulence-associated factors.