A study of the crystallization behavior of isotactic propylene-hexene copolymers prepared with metallocene catalysts is reported. These copolymers crystallize from the melt as mixtures of alpha and gamma forms of isotactic polypropylene (iPP). The amount of gamma form first increases very rapidly with increasing hexene concentration, up to 2-3 mol %, and then rapidly decreases for hexene concentrations higher than 4-5 mol %, and samples with hexene contents higher than 10 mol % (and lower than 15-16 mol %) crystallize from the melt exclusively in the a form. This is due to the fact that at high concentrations hexene units enter to high extents into crystals of alpha form, driven by density increase, favoring crystallization of the alpha form instead of that of the gamma form. These data have confirmed that the crystallization of gamma form of iPP is not only related to the value of the average length of the regular propylene sequences but also related to the different degrees of inclusion of stereodefects and constitutional defects in the crystals of alpha and gamma forms.