Polychlorinated biphenyls (PCBs) typify a class of stable aromatic pollutants that are targeted by bioremediation strategies. In the aerobic degradation of biphenyl by bacteria, the key step of ring cleavage is catalyzed by an Fe(II)-dependent extradiol dioxygenase. The crystal structure of 2,3-dihydroxybiphenyl 1,2-dioxygenase from a PCB-degrading strain of Pseudomonas cepacia has been determined at 1.9 angstrom resolution, The monomer comprises amino- and carboxyl-terminal domains, Structural homology between and within the domains reveals evolutionary relationships within the extradiol dioxygenase family. The iron atom has five ligands in square pyramidal geometry : one glutamate and two histidine side chains, and two water molecules.