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Linear elastic fracture mechanics techniques have been used to study the fracture behaviour of the biodegradable thermoplastic poly(hydroxybutyrate). Both the compact tension and double cantilever beam test geometries have been used and give toughness values in good agreement. The variation in fracture toughness with age after moulding has been monitored. It has been found that the strain energy release rate is reduced during ageing, but the stress intensity factor is increased. The primary change during ageing is found to be in the polymers' stiffness rather than its resistance to fracture.