Twenty-three strains of Pseudomonas aeruginosa isolated from bottled and well waters were biotyped, serotyped and examined by antimicrobial susceptibility, plasmid content, analysis of chromosomal DNA EcoRI ribopatterns (ribotyping) and by a PCR-based procedure. Phenotypic methods showed poor discrimination power between strains, and plasmids were detected in 29% of isolates. Southern blot hybridisation analysis showed genetic diversity between isolates, and six different ribotypes were defined. Arbitrarily primed polymerase chain reaction (AP-PCR) analysis also differentiated between the strains with higher discriminatory power than ribotyping, since 13 different profiles were obtained. Both DNA-based methods are valuable alternatives to traditional typing systems for P. aeruginosa, and AP-PCR could be particularly useful for epidemiological studies.