The ageing of poly( hydroxybutyrate) at room temperature has been studied using wide-angle X-ray scattering. The observed unit-cell parameters and crystal density did not change on ageing. A small increase in crystallinity was observed during the first 32 h storage. No change in crystallinity was detectable after this time. These results are discussed in the light of theories relating to the embrittlement of poly( hydroxybutyrate).