The microorganisms grown on a minimal medium with aniline as a sole source of carbon, nitrogen, and energy were isolated, identified, adn characterized. Pseudomonas acidovorans strain 4Al harbored a plasmid of ca. 230 kilobases, shown to be involved in aniline metabolism. The substrate specificity of aniline-dioxygenase from Pseudomonas testosteroni strain 6Fl was found to be restricted to native form of aniline, indicating that it did not act on aniline derivatives or cationic state of aniline. In addition, this enzyme might not be coordinated with other enzymes of the -keto-adipate pathway. Utilization of aniline was promoted by the addition of other utilizable crbon sources to the bacterial cells of P. testosteroni strain 6Fl in lag phase and in starved condition. It was shown that the NADH level in the cells related to the aniline uptake rate.