The response of the adultTribolium confusumJacquelin duVal to stress imposed by malathion and crowding was investigated. Stress imposed by malathion concentrations of 1.0 ppm or greater with population densities of 75 or more adult beetles per 3 grams of flour was adequate to initiate the production of a gas with resulting death of all beetles. A color change in the flour (white to pink) was a secondary effect resulting from the presence of the quinone gas.