The cytokine transforming growth factor- (TGF- ) plays various functions in the control of Trypanosoma cruzi infectivity and in the progression of Chagas’ disease. When we immunostained T. cruzi-infected cardiomyocytes (after either in vivo or in vitro infections) for TGF- , we observed stronger immunoreactivity in parasites than in host cells. TGF- immunoreactivity evolved during parasite cycle progression, with intense staining in amastigotes versus very faint staining in trypomastigotes. TGF- was present on the surface of amastigotes, in the flagellar pocket, and in intraparasitic vesicles as revealed by electron microscopy. However, no ortholog TGF- gene could be identified in the genome of T. cruzi by in silico analysis or by extensive polymerase chain reaction and reverse transcriptase-polymerase chain reaction studies. Immunoreactive TGF- was most probably taken up by the parasite from the host cell cytoplasm because such an internalization process of biotinylated TGF- could be observed in axenic amastigotes in vitro. These observations represent the first example of a novel mechanism by which a primitive unicellular protozoan can use host cell TGF- to control its own intracellular life cycle.