In vivo metabolism of pyraclofos, (R, S)-[O-l-(4-chlorophenyl) pyrazol-4-yl O-ethyl S-n-propyl phosphorothioate], labeled with $^{14}C$ on the benzene ring was studied in the housefly, Musca domestica. The metabolism of pyraclofos produced five main metabolites: EHP-CHP {1-(4-chlorophenyl) pyrazol-4-yl O-ethyl hydrogen phosphate}, formed by the cleavage of the n-propyl-S-P bond after oxidation of the S atom, CHP {1-(4-chlorophenyl)-4-hydroxy-pyrazole}, formed from cleavage of the P-O-aryl bond, CHP-sulfate, formed from the sulfate conjugate of CHP, CHP-glucose, formed from the glucose conjugate of CHP, and SHP-CHP {1-(4-chlorophenyl) pyrazol-4-yl S-n propyl hydrogen phosphorothioate}, formed from the cleavage of the P-O-ethyl bond. The proposed metabolic pathways of pyraclofos in the housefly are: cleavage of the P-O-aryl bond resulting in CHP and its conjugates, and cleavage of the P-S-n-propyl bond after oxidation of the sulfur atom resulting in EHP-CHP