Isotactic propylene-ethylene (iPPEt) and propylene-butene (iPPBu) copolymers have been prepared with different metallocene catalysts. The different influences of stereodefects (isolated rr triads), ethylene and butene comonomeric units on the crystallization of the alpha and gamma forms of isotactic polypropylene (iPP) have been discriminated. Both iPPEt and iPPBu copolymers crystallize from the melt as mixtures of the alpha and gamma forms. The amount of the gamma form increases with increasing crystallization temperature, comonomer concentration, and content of rr stereodefects. In iPPBu copolymers, the amount of the gamma form decreases for concentration of butene units higher than 10-14 mol % and is always lower than that crystallized in iPPEt copolymers. Butene units, therefore, favor crystallization of the gamma and alpha forms at low and high concentrations, respectively. These data have indicated that the crystallization of the gamma form of iPP is not only related to the value of the average length of the regular propylene sequences < L-iPP >, but is also related to the inclusion of stereodefects and constitutional defects in the crystals of iPP. Very different proportions of ethylene and butene units are included in crystals of the alpha and gamma forms of iPP. Butene units are included indifferently in crystals of the alpha and gamma forms, but probably more easily in the alpha form at high concentrations. Therefore, at low butene concentration, up to nearly 10 mol %, the effect of shortening of the length of regular isotactic propylene sequences prevails and induces crystallization of the gamma form. For butene concentrations higher than 10 mol %, the effect of inclusion of butene units in crystals of the alpha form prevails, producing a decrease of the amount of the gamma form and crystallization of the pure alpha form for butene contents higher than 30 mol %.