The autocatalytic model of nucleation is used to study the crystallization of zeolite ZSM-5 in five systems differing in the Al content of the starting gel. The crystallization kinetics and the nucleation kinetics are examined by SEM and X-ray diffraction. Experimental evidence is presented for the validity of the autocatalytic nucleation model. The two kinds of nucleation (heterogeneous and autocatalytic) processes adequately explain all the crystallization curves.