A two stage growth during isothermal cold crystallization of PHB has been observed in the temperature range 5-40 degrees C when a free surface is present. This growth has been investigated with optical and atomic force microscopy both in-situ and ex-situ. Initially, crystal growth is observed to be composed of lamellae oriented approximately flat-on relative to the free surface. At later stages of growth there can be a change to a distinctly different form of crystal growth that is composed of edge-on lamellae which grow at a substantially higher crystallization rate. This change in growth rate at constant temperature gives rise to curved interfaces between the slower growing flat-on growth and the faster growing edge-on growth. Several possible explanations for this change in growth rate are put forward. (C) 2009 Elsevier Ltd. All rights reserved.