Pseudomonas pseudoalcaligenes POB310 contains genes that encode phenoxybenzoate dioxygenase. The enzyme transforms mono- and dichlorinated phenoxybenzoates to yield protocatechuate that is used as a growth substrate and chlorophenols that are nonmetabolizable. Mass spectral analysis of O-18 metabolites obtained from the protocatechuate 3,4-dioxygenase-deficient mutant, POB310-B1, suggested that the reaction mechanism is a regioselective angular dioxygenation. A cloning vector containing reaction relevant genes (pD30.9) was transferred into Pseudomonas sp. strain B13 containing a modified ortho-cleavage pathway for aromatic compounds. The resultant Pseudomonas so. strain B13-D5 (pD30.9) completely metabolized 3-(4-chlorophenoxy)benzoate. During growth on 3-phenoxy-benzoate, strain B13-D5 (pD30.9) (K-s = 0.70 +/- 0.04 mM, mu(max) = 0.45 +/- 0.03 h(-1), t(d) = 1.5 h, Y = 0.45 +/- 0.03 g biomass . g substrate(-1)) was better adapted to low substrate concentrations, had a faster rate of growth, and a greater yield than POB310 (K-s = 1.13 +/- 0.06 mM, mu(max) = 0.31 +/- 0.02 h(-1), t(d) = 2.2 h, Y = 0.39 +/- 0.02 g biomass . g substrate(-1)).