Two polymerase chain reaction (PCR)-based methods were used for epidemiological typing of Aeromonas hydrophila. Random amplification of polymorphic DNA (RAPD) and enterobacterial repetitive intergenic consensus-PCR (ERIC-PCR) were applied to an outbreak involving seven patients. The epidemiological situation appeared complex; with the exception of two clinical isolates, all gave unique patterns with both techniques. These methods demonstrated nosocomial transmission in one unit and permitted the study to exclude a common environmental source in the hospital. The coincidental clustering of patients infected with A. hydrophila probably resulted from an increased prevalence of aeromonads in waters during summer, although no single RAPD or ERIC-PCR pattern was found among both clinical and environmental samples. RAPD and ERIC-PCR proved to be effective for the epidemiological study of A. hydrophila strains.