The existence and the formation of pre-ordered structures as the initial step during the complex process of polymer crystallization are discussed controversially. Most of the findings and interpretations are based on scattering experiments, which test small density differences between the assumed precursors of the crystals and the surrounding melt. Because of the low contrast the interpretation of experimental results become often speculative. In contrast relaxation experiments are probing motions in the sample and are therefore independent on density contrast. During crystallization, material is transformed from the liquid to the solid state. Consequently, motions typical for a liquid become impossible and do not longer contribute to the measured signal. For pre-ordered structures we expect some changes in mobility too because of the changes in conformation on pre-ordering.We performed dielectric relaxation experiments during isothermal crystallization of PCL. Pronounced effects in epsilon' are observed long before changes in crystallinity can be detected. The observations strongly support the idea of pre-order in the polymer melt before the formation of crystals. (C) 2003 Elsevier Ltd. All rights reserved.