The effect of avian and mouse blood in the gut of Triatoma dimidiata (Hemiptera: Reduviidae) on the biological characteristics of two costarican strains of T. cruzi (TC-2 and TC-4) was evaluated in a murine model. This relationship defined four experimental systems named TC-2 gallina, TC-2 raton, TC-4 gallina and TC-4 raton. Parasites from each system were inoculated intraperitonealy in C 3 H mice and the course of the infections were observed three times a week during 60 days. The observations included parasitemia levels (parasites/mm 3 ), concentration of slender and broad forms, duration of prepatent and patent periods, survival, and cumulated mortality. The infection characteristics in all the systems were similar with parasitemia peaks in the days 23 to 33 post infection and broad forms as the predominant tripomastigote circulating forms in all the systems. Some variables such as prepatent and patent periods did not show statistically significant differences (p < 0.05). However, the survival was prolonged in mice that were infected to parasites associated to avian blood. The evolution of the mortality was slower in these systems. The data suggests that the parasite virulence modulation could be related to the particular blood sources in the vectors.