Pseudomonas aeruginosa strain JB2 was isolated from PCB-contaminated soil using 2-chlorobenzoate as a sole carbon source. The culture was subsequently found to also grow on 3-chlorobenzoate, 2,3- and 2,5-dichlorobenzoate, and 2,3,5-trichlorobenzoate. Additionally, a wide range of other mono- and dihalogenated benzoic acids were utilized by strain JB2 as growth substrates. Cometabolism of 2,4-dichlorobenzoate was also observed. Chlorocatechols were the central intermediates of all the chlorobenzoate catabolic pathways. Degradation of 2-chlorobenzoate was routed through 3-chlorocatechol, whereas 4-chlorocatechol was identified from the metabolism of both 2,3- and 2,5-dichlorobenzote. The initial attack on chlorobenzoates was oxygen-dependent and mediated by a chlorobenzoate-induced dioxygenase. Although plasmids were not detected in strain JB2, a high frequency of spontaneous mutation was observed upon subculturing on glycerol. The mutants were all of the following phenotype; 3-chlorobenzoate+, 2-chlorobenzoate-, 2,3-dichlorobenzoate-, 2,5-dichlorobenzoate-. The loss of ability by the mutants to utilize 2-chloro- and 2,3- and 2,5-dichlorobenzoate coincided with decreased oxygen uptake on these substrates relative to the wild type, indicating that two separate dioxygenases were involved in chlorobenzoate degradation.