A clonal origin for European isolates of antibiotic multi-resistant Pseudomonas aeruginosa serotype O12 has been suggested. This study was designed to assess the value and limitations of several typing methods for the investigation of outbreaks due to this serotype. In Hôpital de Rodez, France, this organism is endemic, and a prospective clinical epidemiological study was undertaken over a 15 month period, encompassing all patients at the hospital from whom P. aeruginosa O12 was isolated. All isolates were examined by auxanogram, antibiogram, phage-typing, electrophoresis of esterases and pulsed-field gel electrophoresis of DNA. The results suggest that (1) the methods used did not clearly differentiate between clinically-related and epidemiologically-unrelated European isolates, (2) in Hôpital de Rodez, while some isolates were likely to have been transmitted from patient-to-patient, most infections or colonizations with this organism were sporadic and their origin is unknown. The limits of typing methods for the investigation of outbreaks of nosocomial infection with multi-resistant P. aeruginosa O12 are emphasized.