Triatoma dimidiata was found in association with 34.6% of the houses in a representative town within the dispersion area of the insect. The mean density over the entire study period was 21.5 insects per house, but it tended to be lower in the majority of the houses. Insects usually were associated with dirt floors, poor sanitary condition of dwellings, and piles of firewood. The infection rate of 3,276 insects for Trypanosoma cruzi was 30.9%. The infection was demonstrated in several synanthropic animals: 25 dogs (9.9%); 3 cats (2.9%); 121 Rattus rattus (30.6%); 1 R. norwegicus (3.8%); and 11 mice (10.7%). Among six species of wild reservoirs from the surrounding bush found infected with T. cruzi, the common opossum (together with the black rat) was considered the most important reservoir of the parasite. Besides the relative abundance and the high infection rate of the opossum (62.5% by direct methods), its natural association with T. dimidiata was likewise demonstrated. Among 1,420 persons examined serologically, 166 (11.7%) were positive; xeno-diagnosis was positive in 14 (2.2%) out of 636 persons, and electrocardiograms (EKG's) were compatible with Chagas' cardiopathy in 60 (6.0%) of 1,006 persons examined. The EKG alterations were significantly higher in serologically positive persons. Studies in a control town, free of vectors, showed complete absence of the infection in both humans and animals.