A total of 42 Pseudomonas aeruginosa strains was isolated previously from clinical sources (27 strains) and from a gasoline-contaminated aquifer (15 strains). Selected strains were subjected to taxonomic tests involving chemical and molecular biological techniques, including membrane fatty acid analysis, phage-sensitivity, growth temperature range, presence of plasmids, and PCR-amplification and sequencing of a species-specific 16S-23S rDNA internal transcribed spacer region. The clinical and environmental isolates formed a coherent taxonomic group with few distinguishing characteristics. Of the phenotypes observed, a consistent difference was the ability of the aquifer strains to utilize gasoline supplied in the gas phase as sole carbon source and, conversely, the inability of the clinical strains to do so. Fourteen of the 15 environmental strains possessed similar-sized cryptic plasmids. The clinical isolates either lacked detectable plasmids or contained plasmids of a different size. The observation that the clinical and environmental isolates of P. aeruginosa were taxonomically indistinguishable is discussed in terms of its relevance to environmental-regulatory guidelines because P. aeruginosa, a known opportunistic pathogen, is a prime candidate for use in bioremediation processes involving deliberate release of this organism to the environment.