A mixture of nine polychlorinated biphenyl (PCB) congeners with four to six chlorine substituents was cometabolized in an aerobic chemostat culture of Alcaligenes sp. JB1, grown on 3-methylbenzoate. Pseudo-first-order biodegradation rate constants (k(b)) were calculated using a steady-state kinetic model. These rate constants varied from 7.8 to 0.44 h-1. Comparable results for exposure to high and low influent concentrations confirmed the assumption of pseudo-first-order kinetics for most of the congeners studied. Alcaligenes sp. JB1 was found to degrade 2,3-dichlorophenyl rings very rapidly and 2,5-dichlorophenyl rings relatively slowly. Certain penta- and hexachlorobiphenyls were degraded faster than some tetrachlorobiphenyls. For example, 2,2',3,3',6,6'-hexachlorobiphenyl was degraded faster than 2,2',6,6'-tetrachlorobiphenyl. On the other hand, 2,2',3,3',4,4'-HCB was degraded much more slowly than 2,2',3,3'-TCB. A control experiment with Pseudomonas fluorescens NCTC10038, a bacterium incapable of PCB (co)metabolism, showed that losses caused by physical or chemical phenomena were negligible.