In Pará State, Brazil, 123 Trypanosoma cruzi stocks were isolated from 12 silvatic mammal species, five silvatic triatomine species and individuals with acute Chagas's disease. 100 T. cruzi stocks were identified as zymodeme (Z) 1, 17 as Z3 and 6 as Z3 with Z1 ASAT character, but none were T. cruzi Z2. Z1 was predominantly isolated from arboreal mammals, especially Didelphis marsupialis; Z3 was mainly found in terrestrial or burrowing mammals, particularly Dasypus novemcinctus and Monodelphis brevicaudata. It is not clear whether gene exchange occurs between the groups designated as zymodemes but the enzymic "distance" between T. cruzi Z1, Z2 and Z3, their different geographical distributions, host associations and local transmission cycles support the view that these zymodemes represent taxonomic units of fundamental epidemiological significance. T. cruzi (Z1) was isolated for the first time from the silky anteater (Cyclopes didactylus).