Malathion. Computer model of a molecule of the organophosphate pesticide malathion. It has the formula C10.H19.O6.P.S2. The atoms are shown as spheres and are colour- coded: carbon (blue), hydrogen (white), oxygen (red), phosphorus (green) and sulphur (yellow). Malathion is a wide- spectrum pesticide used to protect crops from sucking and chewing insects and to treat lice on humans. It belongs to a class of chemicals called cholinesterase inhibitors, which kill by blocking the enzyme acetylcholinesterase, causing paralysis. Large doses of malathion are toxic to humans and can be fatal.