During the summer of 1957, suspected cases of poisoning by cotton insecticides were investigated in the Mississippi Delta area. Blood cholinesterase levels of cases and of workers with maximal exposure to organic phosphorus insecticides were determined. In addition, the decay rates of insecticides and the exposure of workers were measured. Ninety-one illnesses suspected of being poisoned due to insecticides were evaluated. Evidence was found to definitely incriminate organic phosphorus compounds as a cause of illness in two workers in formulating plants. A diagnosis of poisoning must be considered in three pilots. One fatality and one other illness were attributed to the ingestion of chlorinated hydrocarbons. In 7 of 11 crashes or disturbances of the pilot's performance about which enough information was available, three appeared to represent pilot error, one was myocardial infarction, and three--mentioned above--appeared to be mild poisoning with consequent interference with flying ability.