During the St. Louis encephalitis epidemic in Corpus Christi, Tex., in the fall of 1966, malathion was applied by aerial spraying over the city and outskirts to control the vector mosquito. A group of 119 volunteers who received varying degrees of exposure to this spray was studied. This involved comparing pre-spray and postspray cholinesterase activities and compiling histories of exposure and symptoms. A 5% incidence of mild and transient symptoms such as headache, nausea, and weakness was noted in the exposed volunteers, but there were no pathognomonic signs. There was no correlation of symptom frequency or severity with enzyme activity and no statistically or clinically significant change in enzyme activity related to time of spraying. It was concluded that there is negligible risk to human health involved in aerial applications of malathion.