Parathion (O,O-diethyl-O-p-nitrophenyl phosphorothioate) is an organophosphate compound and is one of the most toxic insecticides registered with the U.S. Environmental Protection Agency (EPA). On agricultural crops it was used widely as a broad spectrum insecticide, acaricide, fumigant and nematocide. In 1991 parathion became a Restricted Use Pesticide (RUP). Parathion is well tolerated by crops, does not easily leach through soil and may be degraded by soil microorganisms within weeks. Degradation of parathion by sunlight or liver enzymes results in the active compound paraoxon which interferes with the nervous system through cholinesterase inhibition. Extreme toxicity combined with the ease of exposure has resulted in numerous human and non-pest species deaths. The immediate aerobic metabolic breakdown product of parathion, PNP or p-nitrophenol, was listed as a priority pollutant in 1979 by the US EPA. The main aerobic pathway involves initial hydrolysis of parathion to para-nitrophenol and diethylthiophosphoric acid (middle branch). The oxidation of parathion to paraoxon was observed, but was a minor contribution (right branch). It was not determined whether this reaction was enzyme-mediated or not; it is shown here as being enzyme-mediated, based on the known enzymic oxidation of diethtylthiophosphoric acid to diethylphosphoric acid, though since the reactions are not well-characterized, both reactions cannot be balanced. Under anerobic conditions, parathion was reduced to aminoparathion, which is hydrolyzed to p-aminophenol and diethylthiophosphoric acid (left branch) ([http://www.ncbi.nlm.nih.gov/pubmed/8005|Munnecke & Hsieh, 1976]).