Wellcome Trust, and NIH Grant No. AI 16305: its collaborative activities in Brazil are under the aegis of the Pan American Health Organization.Triatomine species influenced volume of feces produced; Triatoma dimi¬ diata produced the largest volume of feces followed by Panstrongylus megistus. Rhodnius prolixus. and Triatoma infestans. Moreover, stage and sex affected fecal production: 5th-stage female nymphs excreted the largest volume of feces. The amount of blood ingested was significantly correlated with the volume of feces produced in 10 out of 11 experimental groups. Triatome size and volume of feces produced were less closely correlated. Indeed, a "threshold" minimum amount of blood must be ingested before bugs are stimulated to defecate. The defecation habits of triatomines probably influence the vectorial capacity of a triatomine species to a lesser degree than do the density of domestic infesta¬ tions, host affinity, and the degree of adaptation to the domestic habitat.