The biological behaviour of three different T. cruzi samples isolated from patients through xenodiagriosis was observed. Samples were studied considering parameters such as parasi-temic levels, morphology of blood forms, histo-pathological changes, virulence and mice mortality The IDPC 1 sample isolated from a naturally infected patient, who had been treated with benzonidazole, showed low virulence. 83,4% of the animals reached the chronic phase and had low parasitemic levels. In the beginning of the infection trypomastigote forms were slender, but they became broad at the end of the acute phase. Histologically the IDPC 1 sample was myo-tropie as shown by the inflammatory reaction in the muscle cells during the chronic phase. The IDPC 3 sample was isolated from a patient infected by transfusion before chemothe-lapeutic treatment. This sample presented high virulence with total mortality within twelve days and high parasitemic levels with a predominance of slender forms Because of a severe inflammation and parasitism of lymph nodes, spleen and liver that sample was characterized as reticulotropic. The IDPC-2 sample was also isolated from a patient infected by transfusion who had three different chemotherapeutic treatment programs. Tins last sample presented moderate virutence with total mortality within seventeen days. Although the parasitemic levels were low during the evolution of the infection, slender forms were observed Severe systemic inflammation occurred and also intense tissue parasitism This parasitism was higher in mus cles, showing itself to be myotropie. Biological behaviour, in this sample, was similar to the IDPC 3 sample. However two main divergences were observed: low parasitemic levels and a my o tropic tendency. T. cruzi samples, which were isolated from patients considered as therapeutic failures, appear, in this research, to have similar biological behaviour patterns to other strains already studied. However, the former cause an infection which is not so severe in the experimental animal. This suggests that drugs could affect the biological behaviour and as such favouring a better host parasite relationship.