The morphological patterns of blood trypomastigotes from five sylvatic Trypanosoma cruzi strains from Santa Catarina, South Brazil, were studied during the course of infection in experimentally infected mice. A predominance of stout trypomastigotes (greater than 70%) was observed during all over the acute phase in four strains of medium virulence. With the remaining strain, of high virulence, the slender forms predominating at the early infection stage were soon also replaced by stout forms. Since almost all T. cruzi strains displaying predominance of this peculiar morphological pattern have been isolated in South Brazil (Rio Grande do Sul, Santa Catarina) and since there are evidences pointing out to the existence of biological differences among these distinct blood parasites, the authors suggest further investigations of possible correlations between the morphological markers and clinical-epidemiological aspects of Chagas' disease.