A total of 16 colonizing and infecting ofloxacin-resistant Pseudomonas aeruginosa strains and two strains isolated from ventilation equipment fluids, all with similar colonial morphologies and with minor but distinct susceptibility differences, were suspected of belonging to a single outbreak and were studied by arbitrary primer (AP) PCR. Thirteen nonrelated strains were included to evaluate the discriminatory capacity of the technique. AP PCR fingerprinting was compared with serotyping, phage typing, and antibiotic susceptibility testing. AP PCR was performed independently with three different primers. The different AP PCR typing systems yielded almost identical patterns for the epidemic strains and enabled us to differentiate most of the nonrelated strains from each other and from the outbreak strains. The combination of AP PCR typing and the phenotyping techniques that we used enabled us to conclude that an outbreak was occurring. In general, the typeability of AP PCR was greater than those of phage typing and serotyping, while the discriminatory powers of the three methods were comparable.