Diazinon. Computer model of a molecule of the organophosphate pesticide diazinon. It has the formula C12.H21.N2.O3.P.S. The atoms are shown as spheres and are colour-coded: carbon (light blue), hydrogen (white), nitrogen (dark blue), oxygen (red), phosphorus (green) and sulphur (yellow). Diazinon is used to protect crops and garden plants from sucking and leaf-eating insects and as a household insecticide against fleas, flies and ants. It belongs to a class of chemicals called cholinesterase inhibitors, which kill by blocking the enzyme acetylcholinesterase, causing paralysis. Large doses of diazinon are toxic to humans and can be fatal.