In order to establish the course of infection, virulence and pathogenicity, a Paraguayan Trypanosoma cruzi strain isolated from a 12 year old patient in 1989, Balb/c mice were inoculated, and the number of parasites were determined by direct parasitemia (micromethod). Pathogenicity were tested by performing tissue disections and direct microscopic observation. The virulence ranged from 9.10 sobre 4 to 6.10 sobre 5 parasites per ml. Peaks of parasitemia were observed between the days 12 and 14, and wide forms were predominant, but not in the peak days. An amastigote nest was found in intestinal muscle and cardiac tissue in only one mouse from the 12 studied