The fracture behaviour of poly(hydroxybutyrate) (PHB) after high-temperature annealing has been studied using linear elastic fracture mechanics techniques. The effect of the annealing temperature on the polymers＇ fracture toughness both initially and after re-ageing is examined. Annealing at temperatures of 120 degrees C or above is found to result in an improvement in both the critical stress intensity factor and the strain energy release rate measurements of fracture toughness which is largely maintained on re-ageing. A more detailed study of the re-ageing behaviour after annealing at 130 degrees C finds G(c) to remain approximately constant but K-c to increase slightly with age.