AbstractPolyhydroxybutyrate (PHB) is a bacterially produced thermoplastic. Melt‐cast PHB sheets are usually quite brittle. We show that this brittleness is due to cracks within the spherulites. These cracks, which may be either radial or circumferential within the spherulites, depending on the crystallization temperature, form under conditions of no externally applied stress. When the material is strained the cracks grow and join together, leading to brittle failure. It is possible to produce ductile PHB sheets in two ways: first, the cracks may be “healed” by a cold rolling process; second, special crystallization conditions can be used to produce ductile sheets consisting entirely of crack‐free spherulites. The relevance of this work to the ductility of crystalline thermoplastic in general is di