Broad line proton magnetic resonance (PMR) has been used for the study of molecular motion in wholly aromatic thermotropic liquid-crystalline (LC) copolyester at high temperatures up to 340 degrees C. Specimens were thin highly oriented fibers produced from a copolymer of hydroxybenzoic acid and hydroxynaphthoic acid (HBA/HNA). In the vicinity of the thermotropic transition temperature a fine structure of PMR spectra was revealed for as-spun fibers. Annealing leads to smoothing of this structure. Theoretical spectra were calculated and compared with the experimental ones. This made it possible to obtain information on large-scale segmental motion of macromolecules in the LC phase. Heat treatment leads to retardation of the chain cooperative mobility. Dynamic inhomogeneity of the fibers at high temperatures is discussed,