Vanadium has an antibacterial activity against Pseudomonas aeruginosa, especially under conditions of iron limitation. Some degree of resistance to V is inducible by prior exposure to the metal. One mutant (VS1) with a higher sensitivity to V was obtained by transposon mutagenesis of P. aeruginosa PA 59.20, a clinical isolate. This mutant had an insertion in a non-coding region, upstream of a cluster of four genes. Three of them show similarities to genes corresponding to known P. aeruginosa antibiotic efflux systems, including an efflux protein, a membrane fusion protein and an outer-membrane porin. This cluster was named mexGHI-opmD. By allelic exchange, three mutants, ncr (for non-coding region), mexI and opmD were constructed in P. aeruginosa PAO1. Next to V sensitivity, the ncr, mexI and opmD mutants also showed reduced production of elastase, rhamnolipids, pyocyanine, pyoverdine and had reduced swarming motility, phenotypes that are known to be regulated by quorum sensing. All wild-type phenotypes, including growth in the presence of V, were restored by complementation with the complete cluster. The production of N-acyl-homoserine lactones (AHLs) was detected using the Chromobacter violaceum bioassay. Total extracts from the three mutants failed to induce the production of violacein by C. violaceum, although AHLs were detected by TLC and C. violaceum overlay. Violacein production was restored by complementation with mexGHI-opmD. The opmD mutant grew very slowly in LB or CAA medium, indicating that OpmD has an important physiological function for the cell. In conclusion, it is believed that the MexGHI-OpmD pump is probably involved in AHL homeostasis in P. aeruginosa.