Six samples of isotactic polypropylene were examined to study the influence of the thermal treatments and the molecular weights and their distribution on the beta-crystallization of the polymer. The highest amount of the beta-phase was obtained by isothermal crystallization and in correspondence of high average molecular weights and wide molecular weight distributions. Small-angle X-ray scattering pointed out that a partial beta-crystallization seems not to influence the lamellar morphology parameters. Differential scanning calorimetry measurements, at different heating rates, allowed us to confirm that the multiple melting endotherms behavior of the beta-phase is to be correlated to a melting-recrystallization-melting mechanism. (C) 2003 Wiley Periodicals, Inc. J AppI Polym Sci 91: 1008-1012, 2004.