High-molecular-weight poly([R]-(-)-3-hydroxybutyrate) (PHB), a biodegradable thermoplastic, was produced from a suspension culture of transgenic Arabidopsis thaliana plant cells expressing two genes from the bacterium Alcaligenes eutrophus involved in the synthesis of PHB. The molecular structure of the plant-produced polymer was analysed by gas chromatography, mass spectrometry, proton nuclear magnetic resonance spectroscopy, infra-red spectroscopy, spectropolarimetry, differential scanning calorimetry, X-ray diffraction and size exclusion chromatography. The results indicate that the polymer from transgenic plants appears to have a chemical structure identical to that of PHB produced by bacteria. However, the molecular weight distribution of the plant-produced PHB was much broader than that of typical bacterial PHB.