Both the general morphology and the ultrastructure distinguish the giants (cannibals) of Oxytricha bifaria from the normal cells. Beyond the general change of the body shape (i.e. differential increase of the thickness of the body along its major axis), a quantitative variation of some of the ciliary organelles (in somatic ciliature the increase of the number of the serially repeating structures and of the frontal cirri; in oral ciliature the increase of the number of membranelles and of the number of cilia per membranelle; the increase of the ciliary rows of the external paroral membrane) has been evidenced together with a differential modification of certain cortical areas. It can be concluded that it is quite appropriate to speak of cell differentiation referring to the giants of O. bifaria.