The influence of a nonpigmenting beta-nucleating additive in the crystallization of isotactic polypropylene (iPP) is investigated by differential scanning calorimetry and X-ray diffraction. It is found that this additive induces the formation of a very high level of the trigonal modification of iPP. The crystallization and melting behavior of the nucleated systems are studied as a function of the cooling and heating rates and the control of the final temperature during the cooling process. The nucleating agent exerts an important effect on the crystallization temperatures and the polymorphic transitions of iPP, delaying the beta-alpha recrystallization process through an increase in the stability of the trigonal crystals.