A case of hypocholinesterasemia induced by ingestion of trichlorfon is presented. A female patient took 20 gm of this insecticide for the purpose of the suicide. She was brought to the hospital one hour later, and her life was saved by gastric lavage. Cyanosis on lips and nails, pupils with sluggish light reaction and fibrillary muscle twitch were observed upon arrival. Laboratory examination performed on the admission disclosed a serum cholinesterase activity of 0.3deltapH per hour. The enzyme activity was depressed to 0.05 deltapH per hour on the second day of hospitalization. The enzyme activity then increased gradually in the two subsequent weeks and the patient recovered.