Concentrated Malathion was used against Panstrongylus megistus, the principal local vector of Chagas' Disease, in an endemic area in the State of Bahia, Brasil. The efficacy of malathion in the controle of this triatominae was demonstrated after three applications six months apart. The insecticide is lethal to all stages of vector including the eggs. In view of its effectiveness in the control of the triatomine, the absence of any noticeable signs of intoxication in the personnel applying the insecticide and in residents of the treated houses or domestic animals, and the relatively low operational cost, the Authors conclude that malathion is a valuable weapon in campaigns to control the veçtors of Chagas Disease.