AbstractAn analysis of the polymorphic behavior of samples of isotactic polypropylene (iPP) prepared with heterogeneous Ziegler‐Natta catalysts is presented. A comparison with the crystallization properties of iPP samples prepared with a single center homogeneous metallocene catalyst is also shown. Samples of Ziegler‐Natta iPP samples, prepared with MgCl2‐supported catalysts, 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 γ form. The more stereoregular fractions crystallize instead basically in the α form. The relative amount of γ form crystallized from the melt is much lower that that observed in samples of metallocene‐made iPP containing comparable amount of defects. While in the metallocene‐made iPP the distribution of defects along the chains is random, in Ziegler‐Natta iPP samples the majority of 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 α form. This analysis allows concluding that some fractions of Ziegler‐Natta iPP are characterized by chains with a stereo‐block microstructure, consisting in regular isotactic sequences linked to more irregular sequences.