The structure of Entamoeba histolytica trophozoites obtained by aspiration from the human colon and by cultivation in vitro was examined with the electron microscope. Amebae from both sources contained poorly developed endoplasmic reticulum, numerous vacuoles, glycogen particles, small cylindrical electron-dense bodies, and crystalloid structures within the cytoplasm. A well-defined fuzzy coat was present on amebae from the colon. A specific mechanism for the extracellular release of enzymes was not observed. In cultures, a number of dead amebae, some engulfed by living amebae, were seen. It is postulated that lytic enzymes responsible for the pathogenic activity of E. histolytica are released only after the death of some amebae in the colony.