The structure and phase behaviour of the thermotropic main-chain liquid-crystalline copolyester poly[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenylhydroquinon and p-hydrobenzoic acid (molar ration 45/45/10) is determined by X-ray diffraction and differential scanning calorimetry. This random copolyester shows up to 340 degrees C periodic c-axis order in its X-ray diffraction; pattern as well as sharp and diffuse reflections at the equator and other crystallographic directions, indicating well ordered three-dimensional crystalline structure. Two crystalline modifications are observed for quenched and annealed fibres, which are both orthorhombic with slightly different unit cell sizes. Indications of other phases at stilt higher temperatures are reported. They are believed to be a novel mesophase followed by a nematic liquid-crystalline phase and the isotropic melt.