1) - In the blood of a guinea-pig inoculated with blood from an acute case of Chagas' disease in November 12, 1940 a Schizotrypanum appeared, which was found (DIAS and FREITAS) to be morphologically different from usual human strains of Schizotrypanum cruzi by its "mean nuclear index" (=1.4) and its mean total length (=20-21 μ). 2) - Large crithidiae with the morphology of Trypanosoma rangeli TEJERA and metacyclic forms of Schizotrypanum have been observed in the faeces of laboratory-bred Rhdnius prolixus, sixty-five days after feeding on the same patient on June 30, 1941. 3) - A strain of Schizotrypanum, identical to the one observed in the guinea-pig injected with the patient's blood, has been isolated through the inoculation of intestinal contents of the so infected Rhodnius prolixus. 4) - In the gut of Rhodnius prolixus and other species of kissing bugs which fed upon animals infected with that strain, only cruzi-like flagellates have been encoutered, not rangeli-like ones. 5) - The hypothesis is suggested that Trypanosoma rangeli is a pathogenic human Schizotrypanum and that certain of its forms in the insect vector are rather inconstant ones, the conditions that determine their appearance being unknown.