Methyl parathion hydrolase (MPH, E.C.3.1.8.1), isolated from the soil-dwelling bacterium Pseudomonas sp. WBC-3, is a Zn(II)-containing enzyme that catalyzes the degradation of the organophosphate pesticide methyl parathion. We have determined the structure of MPH from Pseudomonas sp. WBC-3 to 2.4 angstroms resolution. The enzyme is dimeric and each subunit contains a mixed hybrid binuclear zinc center, in which one of the zinc ions is replaced by cadmium. In both subunits, the more solvent-exposed beta-metal ion is substituted for Cd2+ due to high cadmium concentration in the crystallization condition. Both ions are surrounded by ligands in an octahedral arrangement. The ions are separated by 3.5 angstroms and are coordinated by the amino acid residues His147, His149, Asp151, His152, His234 and His302 and a water molecule. Asp255 and a water molecule serve to bridge the zinc ions together. MPH is homologous with other metallo-beta-lactamases but does not show any similarity to phosphotriesterase that can also catalyze the degradation of methyl parathion with lower rate, despite the lack of sequence homology. Trp179, Phe196 and Phe119 form an aromatic cluster at the entrance of the catalytic center. Replacement of these three amino acids by alanine resulted in a significant increase of K(m) and loss of catalytic activity, indicating that the aromatic cluster has an important role to facilitate affinity of enzyme to the methyl parathion substrates.