The experimentally estimated ’normalized’ rate constants of the bulk crystallization of poly(L-lactic acid) and natural rubber at different temperatures have been used in order to test different equations describing the temperature dependence of the normalized crystallization rate constants. Ten of these expressions are variants of the well-known Turnbull-Fisher relationship k = k(0) exp[-U*/R(T - T-infinity)] exp[K-g/T(Delta T)f] which differ essentially by the assumed number of fixed parameters [one or two Williams, Landel, and Ferry (WLF) constants i.e., U* = C-1 and/or C-2]. Two additionally used equations are expressions derived on the basis of the above mentioned general relation, which introduce T-g instead of T-infinity = T-g - C-2 as the low temperature limit of crystallization and two energies of activation, for nucleation and crystal growth, respectively. It is shown that the temperature dependence of the normalized bulk crystallization rate constants is more accurately described if three parameters (beside k(0), U*, and K-g or the respective activation energies) are assumed to be adjustable and when instead of the widely accepted expression for the low temperature limit, T-infinity = T-g - C-2(K) with C-2 = 51.6 or 30 degrees C, respectively, T-g(i.e., C-2 = 0 degrees C) is considered. A qualitative explanation for this finding is suggested in terms of the increasing mobility of the crystallizing segments at temperatures above T-g, taking into account that generally the glass transition is considered to be a ’freezing in’ process.