The induction period of Oxytricha bifaria, namely the time lag elapsing between the beginning of the inducing (= overcrowded) conditions and the formation of the first giant was studied. It was found that this period consists of at least three successive steps: (a) the "early activation window", roughly corresponding to the first third of the induction period, during which the cell to cell contacts triggering the differentiation must occur; (b) the "late activation window", which corresponds to the central part of the induction period and during which a specific protein synthesis occurs. These two steps end in the "activation" of Oxytricha, which is now ready for the third phase, (c) "predation" which occurs during the last third of the induction period, when the activated, potential giants actually engulf their preys.