Following an outbreak of illness among peach harvesters, 186 peach orchard workers were studied in relation to pesticide application practices and fruit harvesting procedures representative of the orchards in which they worked. It was necessary first to establish a diagnosis of organic phosphate poisoning and then to associate observed illness with the intensity of parathion application. Information obtained revealed that, although parathion could easily be recovered from all elements of the orchard environment, it was not present in amounts sufficient to account for the observed illness. This inconsistency suggested the presence in the spray residue of a compound evolved from parathion alteration which was considerably more toxic than parathion, but identifiable by routine analytical procedures only as parathion. Paraoxon was considered a likely suspect and was postulated as a prime cause of the outbreak.