Because the presence of Trypanosoma rangeli in Argentina and Brazil has not been confirmed, a serach was carried out in man and triatomine insects. Fifty-nine of 207 persons (28.5%) were infected when studied with one or more xenodiagnosis (40 Triatoma infestans/xenodiagnosis); 0.1% to 13% of 7,821 bugs' feces and 4% of 875 dissected midguts showed T. cruzi in Giemsa stained smears. One of 6,980 hemolymph samples and 1/875 salivary glands showed few flagellates which may have originated in the gut. They were not found in stained smears. Thirteen percent of 188 wild-caught domiciliary T. infestans showed only T. cruzi in the gut and feces. Although the presence of T. rangeli could not be demonstrated, the study indicates that examination of pooled hemolymph and random samples of salivary glands and midguts can be carried out together iwth the usual examination of extracted feces in any species of triatomine, including those with salivary glands lacking the pink color present in the genus Rhodnius. Unless this procedure is widely used it will not be possible to differentiate T. rangeli and other possible trypanosome infections from those of T. cruzi nor to determine, therefore, the true prevalence of Chagas' disease in a given human population.