Various Pseudomonas aeruginosa PAO1 NfxB mutants were isolated on agar plates containing cefpirome and ofloxacin. They were classified into type A and type B, based on the degrees of changes in their susceptibilities. Type A mutants were four to eight times more resistant to ofloxacin, erythromycin, and new zwitterionic cephems, i.e., cefpirome, cefclidin, cefozopran, and cefoselis, than was the parent strain, PAO1. In contrast, type B mutants were more resistant to tetracycline and chloramphenicol, as well as ofloxacin, erythromycin, and the new zwitterionic cephems, than was PAO1, and they were four to eight times more susceptible to carbenicillin, sulbenicillin, imipenem, panipenem, biapenem, moxalactam, aztreonam, gentamicin, and kanamycin that was PAO1. The changes in susceptibilities of type B mutants were greater than those of type A mutants. The susceptibilities of both type A and type B mutants were restored to the level of PAO1 by transformation with plasmid pNF111, which contained the wild-type nfxB gene, demonstrating that they are NfxB mutants. Immunoblot analysis with a monoclonal antibody to OprJ revealed that type B mutants produced larger amounts of outer membrane protein OprJ than did type A mutants and that PAO1 produced an undetectable amount of it. Moreover, transconjugants obtained with the different types of NfxB mutants as the donor strains showed almost the same phenotypes as the corresponding donor strains. These results suggest that there are at least two nfxB mutations that show different phenotypes and that production of OprJ is associated with changes in susceptibilities of NfxB mutants.