Xanthobacter flavus 14p1 used 1,4-dichlorobenzene as the sole source of carbon and energy but did not grow on other (chloro)aromatic compounds. 1,4-Dichlorobenzene was attacked by a chlorobenzene dioxygenase, and the intermediate chlorocatechol was metabolized by the modified ortho pathway. All enzymes necessary to convert 1, 4-dichlorobenzene to 3-oxoadipate showed a low substrate specificity and also accepted the respective intermediates of chlorobenzene or 1, 3-dichlorobenzene degradation. Of the three compounds chlorobenzene, 1,4-dichlorobenzene, and 1,3-dichlorobenzene, the latter was the most toxic for X. flavus 14p1. Furthermore, 1,3-dichlorobenzene did not induce chlorocatechol 1,2-dioxygenase activity of the organism. Chlorobenzene, however, induced chlorocatechol 1,2-dioxygenase, dienelactone hydrolase, and maleylacetate reductase activities. As demonstrated by chloride release, also chlorobenzene dioxygenase, chlorobenzene cis-dihydrodiol dehydrogenase, and chloromuconate cycloisomerase activities were present in chlorobenzene-induced cells, but chlorobenzene failed to support growth. Presumably a toxic compound was formed from one of the intermediates.