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Triatoma brasiliensis is an important vector of Trypanosoma cruzi, commonly found in semi-arid areas of north-eastern Brazil. T cruzi (TcI) is a widely distributed genotype in all biomes of Brazil. To evaluate selective pressures exerted by a vector species on the development of TcI derived from a different biome (Atlantic Rainforest), T brasiliensis larvae were infected with the MDID/BR/1994/C48 isolate. Parasite densities of T cruzi were determined in three regions of the gut at 3, 5 and 10 days after feeding. Percentages of the different stages of the flagellate were identified in Giemsa stained smears. The TcI isolate possessed always significantly higher densities in the rectum than in the small intestine. Epimastigotes reached their highest percentage at 3 days after feeding in the small intestine and trypomastigotes at 10 days after feeding in the rectal wall. Additionally, high metacyclogenesis rates in the T brasiliensis gut showed competence of this TcI strain to complete its life cycle in this unfamiliar vector species. (C) 2008 Elsevier B.V. All rights reserved.