Seven samples of isotactic polypropylene were examined to study the influence on the formation of the gamma crystalline phase of possible regiodefects along the chain. Wide-angle X-ray diffraction allowed the determination of the percentage of the gamma phase in the samples and C-13-NMR spectroscopy was used to correlate the development of the gamma phase with the existence of regioirregular structural units along the chain. Furthermore, it was possible to appraise the contributions given by the different families of lamellae to the small-angle X-ray diffraction patterns.