In samples of isotactic polypropylene, the influence of the nucleation agent chinacridine permanent red E3B on the crystallization rate and content of α and β modifications and on the rate and mechanism of the β →αtransition was investigated using x-ray diffraction, DSC microcalorimetry, and electron microscopy. It was found that, though E3B increases the content of the β phase, it is also a very efficient nucleation agent for the α-phase crystallization. The β → α transition occurring above 140°C could be characterized by a gradual melting and annealing of the β phase, accompanied by a surface crystallization of the α phase on lateral faces of β lamellae by a mechanism similar to cross-branching of α lamellae.