Although aromatic compounds are most often present in the environment as components of complex mixtures, biodegradation studies commonly focus on the degradation of individual compounds. The present study was performed to investigate the range of aromatic substrates utilized by biphenyl- and naphthalene-degrading environmental isolates and to ascertain the effects of co-occurring substrates during the degradation of mono-aromatic compounds. Bacterial strains were isolated on the basis of their ability to utilize either biphenyl or naphthalene as a sole source of carbon. Growth and transformation assays were conducted on each isolate to determine the range of substrates degraded. One isolate, Pseudomonas putida BP18, was tested for the ability to biodegrade benzene, toluene, ethylbenzene and xylene isomers (BTEX) individually and as components of mixtures. Overall, the results indicate that organisms capable of growth on multi-ring aromatic compounds may be particularly versatile in terms of aromatic hydrocarbon biodegradation. Furthermore, growth and transformation assays performed with strain BP18 suggest that the biodegradation of BTEX and biphenyl by this strain is linked to a catabolic pathway with overlapping specificities. The broad substrate specificity of these environmental isolates has important implications for bioremediation efforts in the field.