We performed serological and pathological studies on 495 patients with Chagas' disease from different areas of Bolivia. Eighty-nine Trypanosoma cruzi strains, isolated by xenodiagnosis, were characterized by 12 isoenzyme loci and were related to the presence of cardiac changes and enteric disease with megacolon. There was a high heterogeneity of human zymodemes, presenting evidence of 2 predominant zymodemes genetically dissimilar from each other and ubiquitous in Bolivia. The frequencies of these predominant zymodemes among strains from patients were compared to strains from triatomine bugs previously studied. We observed mixtures of different zymodemes within the same patient, a phenomenon seen previously in Bolivian patients. There was no apparent difference of pathogenicity between the 2 more frequent zymodemes isolated from humans.