In the present paper details are given on the discovery of Trypanosoma conorrhini (DONOVAN, 1909) (syn. Crithidia conorrhini DONOVAN, 1909; Trypanosoma boy lei LAFONT, 1912) in its insect vector, Triatoma rubrofasciata (DE GEER, 1773) and in its vertebrate host, the rat (Rattus rattus diardi). The universal geographic distribution of Triatoma rubrofasciata is recorded. The writers report the finding in Rio de Janeiro, Brazil, of an adult example of T. rubrofasciata infected with flagellates that are referable to Trypanosoma conorrhini. The close resemblance of certain forms of this flagellate to the insectan forms of Schizotrypanum cruzi is emphasized. Metacyclic trypanosome forms of each species, however, may be distinguished; those of Trypanosoma conorrhini are shorter and their nucleus is closer to the parabasal body than in Schizotrypanum cruzi. Data concerning the measures of 100 metacyclic forms of each species are given: The mean total length of T. conorrhini was 13.88U and that of Schizotrypanum was 19.85µ. The experimental evolution of T. conorrhini has been easily obtained in Triatoma infestans, Triatoma vitticeps, Rhodnius prolixus and Panstrongylus megistus. By the intraperitoneal inoculation of white mouse, rat and Rhesus monkey light infection were produced. Parasites were rarely seen in fresh blood preparations, but could be easily demonstrated in the peripheral blood of apparently recovered mice untill 53 days after the inoculation, by "xenodiagnosis". The blood form of Trypanosoma conorrhini is a large trypanosome with long and pointed posterior end, a sub-central parabasal body with a particular pot-shaped structure just in front of it, and a wide ondulating membrane. A few hours after the peritoneal inoculation of the gut contents of infected Triatoma rubrofasciata parasites may appear in the blood, where in 3-4-days they reach the large size of the typical trypanosome form. The number of trypanosomes in the peripheral blood depends upon the number of parasites in the inoculum. It is interesting to note that so far division forms of T. conorrhini have never been found in the blood or tissues of infected animals. Sub-inoculation of the blood forms usually do not succeeded beyond the first, and even the first sub-inoculation result in very light infection. Cultures from the infected animal blood have not yet succeded (MORISHITA, 1935). So far Trypanosoma conorrhini appear to be the only species of Trypa-nosoma transmitted by reduviid-bugs; a list is given of trypanosomes which failed to develop in such bugs on experimental conditions.