A pseudomonad isolated from activated sludge produced a black humus-like material from a commercial PCB mixture, Aroclor 1221. A similar product was produced from a more highly chlorinated PCB, Arochlor 1242, in the presence of biphenyl. The meta-oxidation product from the bacterial metabolism of Aroclor 1242 reacted with catechol (1,2-diphenol), yielding a dark product similar to that produced by the metabolism of Arochlor 1221. It is suggested that the reaction involves the oxidative co-polymerization of a meta-oxidation product derived from a 2,4’-substituted chlorobiphenyl with a diphenol derived from biphenyl. The possible significance of such polymerization reactions in immobilizing PCB components in the environment is discussed.