We show here that HeLa cell microfilaments can be stained by phalloidin at the sites of invasion of Trypanosoma cruzi trypomastigotes, one of the infective stages of this protozoan parasite. Concurrently, a projection of the HeLa cell plasmalemma encircles invading parasites. This plasmalemma projection is further internalized and entire membrane protrusions containing parasites are found within cytoplasmic vacuoles of the host cell. Neither the microfilament staining around invading parasites nor the plasmalemma extension is inhibited by cytochalasin D, a drug that is unable to prevent trypomastigote entry into HeLa cells. The internalization of the membrane expansion, however, is blocked by the drug. These novel observations indicate that although the driving force for T. cruzi penetration comes from the parasite, the cortical target cytoskeleton of the target cell is concomitantly modified. The molecular characterization of this phenomenon may provide a new insight into the understanding of the mechanisms involved in the active penetration of T. cruzi into mammalian cells.