Pseudomonas putida can utilize a simple chlorinated compound 3-chlorocatechol (3-clc) through elaboration of a plasmid pAC27 encoded pathway. The clc genes are clustered as an operon termed clcABD. The positive regulatory gene clcR maps close to but is transcribed divergently from the clcABD operon. A similar genetic organization for catechol (Cat) degradation has been shown, where the catB gene of the catBC operon and its divergently transcribed catR regulatory gene show appreciable homology to clcB and clcR. This suggests that clc genes evolved by diverging from an extant, regulated catechol pathway. In contrast, a strain of P. cepacia (AC1100) was isolated from a chemostat under strong selection in the presence of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). This strain is characterized by: (1) marked genetic instability specific to the tft genes of the 2,4,5-T pathway, (2) several copies of the insertion sequence, RS1100, and (3) lack of detectable hybridization of either RS1100 or of the chq locus in the tft pathway with DNA from several species of pseudomonads.