We have cloned blood trypomastigotes from infected mice and found that Trypanosoma cruzi strains are composed of heterogeneous populations that dramatically vary (more than 100 fold) in their abilities to attach to and enter rat heart myoblasts. Trypomastigote clones were distinctively separated into highly and weakly infective groups presenting higher and lower rates of attachment to myoblasts, respectively. Each trypomastigote clone maintained the same profile of attachment and internalization into heart myoblasts when tested at different periods of time. This pattern did not change when the parasites were incubated in fresh medium before being exposed to heart myoblasts. Highly and weakly infective clones show differences at the cell surface level, particularly with regard to a 83 kDa glycoprotein. We have identified this 83 kDa glycoprotein as the parasite membrane ligand that specifically binds to rat heart myoblasts. The binding of the biotinylated 83 kDa to myoblasts is inhibited by cold excess in Western blots, as indicated by laser densitometry. In addition, the specific binding of this molecule to myoblasts is saturable and is greater in highly than in weakly infective trypomastigote clones. Highly invasive trypomastigote clones express this glycoprotein in more abundance on their surface than weakly infective trypomastigote clones. These results indicate that the 83 kDa glycoprotein present on the surface of T. cruzi trypomastigotes mediates the attachment of the parasite to heart myoblasts.