During the period from October 1996 to October 1998, captures of mammals were made in two places with different ecological characteristics, La Matica and Tintinal, searching for reservoirs of Trypanosoma cruzi. A total of 513 animals, belonging to 10 different genera, were captured. Blood was extracted from them by cardiac puncture, to carry out examinations in fresh and culture in blood agar base medium. Xenodiagnosis was also carried out on a limited number of animals. Two different species were found naturally infected with T. cruzi, opossums (Didelphis marsupialis) and black rats (Rattusspp.). In D. marsupialis the presence of T. cruzi was determined in 9 of the 31 animals studied (29.03 por ciento). Rattus was found to be infected in 4.41 por ciento (3/68). Five cultures (2 from D. marsupialis and 3 from Rattus spp.) were characterized as T. cruzi by means of the kDNA restriction pattern and hybridization with species-specific probes. When we studied the D. marsupialis according to the place of capture, we found that the infection in Tintinal was present in 57.14 por ciento (8/14) of the animals studied, while in La Matica it scarcely represented 5.88 por ciento (1/17). These differences were statistically significant.(p menor que 0.05). Finally, the inclusion of a ôprevalence index of T. cruzi in reservoirsõ is suggested, as an additional measure to predict the real situation of an endemic area