Studies of the temperature dependence of spherulitic growth rates, in the context of the Lauritzen-Hoffman secondary nucleation theory, and investigations of the crystallization time and temperature dependence of the melting behavior, in the context of the nonlinear Hoffman-Weeks (HW) extrapolation, are reported for isotactic polypropylene prepared with Ziegler-Natta catalysts. The quantitative agreement between estimations of the equilibrium melting temperature, T-m, through an analysis of spherulitic growth rate data and use of the nonlinear HW approach provides strong support for the latter method. The results of these studies indicate specifically that T-m (it-PP, alpha-phase) is in the vicinity of 212-215 degrees C, some 30 degrees C above the most commonly quoted value. This new value of the equilibrium melting temperature of it-PP leads to predictions of the crystallization temperature dependence of the initial lamellar thickness and of the correlation between the lamellar thickness and observed melting temperature, which are in good agreement with the limited morphological information available. The present studies confirm the existence of a regime II/III transition at T-x = 139.5 degrees C. The results of this study furthermore cast some doubts as to the current value of the small-angle X-ray scattering technique for the analysis of morphological parameters in quiescently crystallized isotactic polypropylene.