The organophosphorus insecticide hydrolase (OPH) gene of Arthrobacter sp. strain B-5, isolated from turf green soil was cloned into Escherichia coli JM109. Three clones, termed EpB511, EpB521 and EpB531, exhibiting OPH activity were obtained. However, these three clones showed lower OF-degrading ability than strain B-5. A 7.7-kb inserted fragment of the plasmid pB521 harbored by EpB521 was subcloned, resulting in construction of a plasmid, pB526, carrying the 2.6-kb inserted fragment with OF-degrading ability. In this sequence, an open reading frame (ORF) that encodes a 43,607 Da plolypeptide composed of 415 amino acids was identified. The N-terminal amino acid sequence deduced from the nucleotide sequence was identical to that of purified OPHs. The deduced amino acid sequence was compared with the sequences in the data bank and a 58.1% amino acid identity was found with the aryldialkylphosphatase from Nocardia sp. strain B-l, an enzyme that possesses catalytic functions similar to OPH.