This article provides a thorough analysis of the crystallization process of poly(butylene terephthalate) (PBT). isothermal crystal growth rates have been experimentally measured between 459 and 491 K, and analyzed according to the Hoffman and Lauritzen theory. A II-III regime transition occurs at 484 K, accompanied by a change in crystal morphology. By means of a non-linear fitting procedure, precise values of the nucleation constant K-g were obtained. The lateral surface free energy sigma was calculated and, from this, the alpha parameter of the Thomas-Staveley expression was obtained.