A strain of T. cruzi was isolated in baby white mice from feaces of specimens of Triatoma infestans captured at Vitichi, Bolívia. This strain, which behaves in a peculiar way in aglutination tests, according to Siqueira, Ribeiro and Fernandes (1973), is highly pathogenic for mice, infecting one hundred per cent of the animais inoculated either with metacyclic trypanosomes from triatomines or with blood from donors with acute infections. Blood trypanosomes in mice have a mean total lenght of 23.37 μ and a mean nuclear index of 0,94. In mice killed during the acute fase of the infection leishmanial forms are abundant, specially in the heart. The infection in mice is severe with short prepatent period, high parasitemia and high mortality rate. Mice recovered from an infection by the Bolívian strain have a high resistance against reinfection by the Y strain of T. cruzi. The Bolivia-strain is easily cultivated in monophasic and diphasic media and infects regularly several species of triatomines tested