A comparative laboratory study of feeding and defecation behavior of three species of tritomines (Rhodnius prolixus, Triatoma infestans and T. dimidiata) indicated evident differences among the species and among the different stages of same species. Time required for a full blood meal was related to the size of the specimen. Insects required an average of 11-28 min for engorgement although some finished within 10 min. T. dimidata frequently interrupted the act of feeding, a probable explanation of the higher number of defecations before finishing a blood meal observed in the species. R. prolixus was superior to the other two species in number of defecating insects and in rapidity and frequency of defecations within a given time. T. dimidiata was inferior in all three parameters and T. infestans was intermediate. Males of all species tended to be less effective. A "defecation index" is proposed for comparing this different behavior in triatomine specimens under standard conditions. Effectivity of the insects according to the measured parameters is discussed in relation to the prevalence of Chagas' disease in those areas where they are principal vectors.