A focus of Chagas' disease transmitted by Triatoma infestans is described in the lowlands of the Fluminense area, Municipality of Caxias, State of Rio de Janeiro. Out of 33 dwellings investigated, the presence of triatomides was noted in 18, various of them infected by T. cruzi. A sorological survey made in 110 people living in this area revealed a positivity in the complement fixation test for Chagas' disease in six, three of which. had positive xenodiagnosis for T. cruzi. Out of six patients with C.F.T. positive, including one with a positive xenodiagnosis, three have always lived in the focus area. This coula, therefore, prove the autochtonal transmission of the diseuse. The authors call attetion to the break in the "ecological barrier" in the known dispersion of T. infestans, with adaptations to human dwellings in a region where the climatic conditions (temperature, humidity) etc.) should be disfavorable to the development of this species. The most important epidemiological fact was lhe adaptation of one species with a great capacity as a vector of Chagas' disease in an area with a dense population and very low social, economic and sanitary conditions. Despite the fact that T. cruzi appears to be of low pathogenicity, the authors believe that it is necessary to carry out a greater number of experimental studies of the isolated samples.