Organic solvent-tolerant mutants ofPseudomonas aeruginosaselected in the presence of hexane exhibited increased resistance to a variety of structurally unrelated antimicrobial agents, including beta-lactams, fluoroquinolones, chloramphenicol, tetracycline, and novobiocin, a phenotype typical ofnalBmultidrug-resistant mutants. Western immunoblotting with antibodies specific to components of the three known multidrug efflux systems inP. aeruginosademonstrated that the solvent-tolerant mutants displayed increased expression of the MexAB-OprM system and decreased expression of the MexEF-OprN system. Sequence analysis ofmexR, the repressor gene ofmexAB-oprMefflux operon, identified a nonsense mutation and a point mutation in themexRgenes of two solvent-tolerant mutants. These results emphasize the importance of the MexAB-OprM efflux system in organic solvent tolerance and the ability of environmental pollutants to select bacteria with a medically relevant antibiotic-resistant phenotype.Key words:Pseudomonas aeruginosa, organic solvent tolerance, multidrug resistance, MexAB-OprM efflux pump,mexRgene.