A comparative analysis of the polymorphic behavior of samples of isotactic polypropylene UP) prepared with heterogeneous Ziegler-Natta catalysts and with a single-center homogeneous metallocene catalyst is presented. Different samples of Ziegler-Natta iPP, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the gamma form. The more stereoregular fractions crystallize instead basically in the a form. This confirms that, even in the case of Ziegler-Natta iPP samples, the gamma form may develop by melt-crystallization at atmospheric pressure in fractions containing a high concentration of defects. The relative amount of gamma form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the gamma form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler-Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler-Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler-Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the a form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the gamma form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of gamma form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler-Natta iPP are characterized by chains with a stereoblock microstructure, consisting of regular isotactic sequences linked to more irregular sequences. The latter contain the major part of stereodefects mainly consisting in isolated rr triads, r diads, and longer...rrrr... syndiotatic sequences. The hypothesis of a stereoblock microstructure for some of these less stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.