Azinphos-methyl (Guthion) is a broad spectrum organophosphate insecticide manufactured by Bayer CropScience, Gowan Co., and Makhteshim Agan. Like other pesticides in this class, it owes its insecticidal properties (and human toxicity) to the fact that it is an acetylcholinesterase inhibitor.

Azinphos-methyl is a neurotoxin derived from nerve agents developed during World War II. In the US, it is registered for use on select nut trees, vegetable crops, and fruit trees. It is not registered for consumer or residential use. It has been linked to health problems of farmers who apply it, and the U.S. Environmental Protection Agency (EPA) considered a denial of reregistration, citing, “concern to farm workers, pesticide applicators, and aquatic ecosystems". After settling a 2004 lawsuit brought by the United Farm Workers of America and other groups, the EPA accounced it would begin phasing out the remaining uses of the pesticide in 2007 with all uses ending in 2012. In January 2007, the suit was reopened, with the plaintiffs seeking a quicker phaseout.