Our observations suggest that T. infestans and P. megistus were easier to infect with Trypanosoma cruzi after feeding on infected dog than two species of Rhodnius. In armadillo better infection rates for T. infestans and T. brasiliensis were obtained while those of species of Rhodnius was low. Also, P. megistus recorded high levei of infection but this results could not be compared. During two months after inoculated, beginning at the 5th day, trypanosomes were detected through xenodiagnosis in armadillos. In mice, four species of triatomids used simultaneously with Rhodnius had a higher infection rate, but Rhodnius was more susceptible than other species. Eight out 10 mices inoculated with two different virulent strains of T. cruzi died in a period of four months, two surviving for more than 11 months. The per- centage of positive xenodiagnosis of these mices during the first two months after inoculation varied from 28-96 %. The positivity of xenodiagnosis could not always be correlated with patent parasitaemia, however xenos were most positive during the major phases of parasitaemia. The infection rates of triatomids fed on different sources (dog, armadillo and mice) of T. cruzi varied according to the infected host. By the exposed data it could be thought that the triatomid susceptibility would be dependant on several factors, among thern the strain of trypanosomes, the species and the stage of the triatomid, and also the host species.