SUMMARYThe toxicity of malathion (2.5 % solution in kerosene) was determined by topical application to the hindwing base of mature adults ofSchistocerca gregariaandLocusta migratoria migratorioidesand of immature adults ofS. gregaria.Mature adults ofL. m. migrator ioideswere slightly more resistant to malathion than those ofS. gregaria.Immature adults ofS. gregariawere approximately as susceptible as mature ones (at LD89level) when the criterion for toxicity was taken as the ultimate death of the dosed insects, but the symptoms of poisoning developed earlier and more strongly in the immature insects.Penetration of malathion was least in the matureL. m. migratorioidesand highest in the immatureS. gregaria.The higher rate of activation of malathion in the immatureS. gregariawas, however, counterbalanced by a higher rate of hydrolysis.The toxicity reflected the rate of accumulation of malathion and/or mala‐oxon inside the insect bod