The ED50 for moulting inhibition by injected azadirachtin A is for fourth instar larvae of all the triatomines, Triatoma vitticepes, T. pseudomaculata, T. maculata, T. brasiliensis, T. lecticularis, T. matogrossensis, T. infestans, Rhodnius prolixus. R. neglectus, R. robustus, Panstrongylus megistus, and P. herrera in the range of 10-25 ng/larva. In Rhodnius prolixus, the survival of T. cruzi was studied after treatment with the drug. If the trypomastigotes were fed in presence of 1.0 microgram azadirachtin A/ml blood, the number of parasites decreased near to the limit of detection within 30 days. If the drug was applied 20 days after T. cruzi infection, it still completely abolished the parasite in the host's gut within the subsequent 20 days. The same holds true if the insect larvae were pretreated with azadirachtin A 20 days before the subsequent infection with T. cruzi. Azadirachtin A. if applied at the dose of 1 microgram/ml, did not affect the hemolytic activity of the crop contents or the proteinase content of the intestine. A parallel between azadirachtin effects on the hormone balance of the host and growth inhibition of the parasite is discussed on the basis of the present results.