Multidrug-resistant derivatives of Pseudomonas aeruginosa PAO1 were obtained after stepwise selection on tetracycline or erythromycin. Two phenotypes were generated. The tetracycline-resistant mutant (TETR) was phenotypically similar to OprM-overexpressing strains. This group displayed cross-resistance to quinolones, chloramphenicol, and all beta-lactams tested except imipenem, with no changes in the erythromycin MICs for the strains. Sodium dodecyl sulfate-polyacrylamide gels showed the overproduction of an outer membrane protein in the range of 50 kDa and a 46-kDa inner membrane protein. The erythromycin-resistant mutant (ERYR) kept its susceptibility to all beta-lactams tested with the exception of cefpirome, but it was resistant to chloramphenicol, quinolones, and tetracycline and was hypersusceptible to imipenem. This mutant also exhibited overexpression of a 50-kDa outer membrane protein that was different from OprM and of a 43-kDa inner membrane protein. The phenotype of ERYR was comparable to those of OprK- and OprJ-overexpressing strains. These strains were therefore classified as the OprK-like group. Transduction of the oprK::omega-Hg mutation of strain K613 (K. Poole, K. Krebes, C. McNally, and S. Neshat, J. Bacteriol. 175:7363-7372, 1993) into the multidrug-resistant strains resulted in the loss of multidrug resistance and the acquisition of hypersusceptibility in the OprM group, while the phenotype of the OprK-like group was unaffected. These experiments demonstrated the existence of two genetically distinct efflux systems in P. aeruginosa. The identities of the operons encoding the two efflux systems and their physiological roles are discussed.