The structure of a wholly aromatic copolyester fiber containing 60 mol% p-hydroxybenzoic acid (PHB), 20 mol% 4-4'-dihydroxydiphenyl, 15 mol% terephthalic acid and 5 mol% isophthalic acid was studied by means of electron microscopy and wide-angle X-ray diffraction. The X-ray diffraction pattern of the wholly aromatic copolyester fibers showed two sets of diffractions: one set was composed of sharp spots which arose from relatively high crystalline phase. The crystal structure analyzed by these spots was orthorhombic and the lattice dimensions were a = 0.869 nm, b = 0.510 nm, c = 1.20 nm and ρ = 1.50 g/cm3. Another set was characteristic of streaks on the meridian extending parallel to the equator. X-ray scattering intensity distribution on the meridian was calculated as the square of Fourier transform of random chain model. Comparison of this intensity distribution with the observed meridional maxima concluded that the streaks were due to rather disordered chains with a PHB content of less than 50%. Dark field image (DFI) taken from the meridional 002 reflection showed that slender crystallites were distributed over the whole visual field, oriented parallel to the fiber axis. On DFI from the equatorial 200 reflection, some of these crystallites were also observable, forming groups that distributed randomly in the field. All crystallites belonging to the same group co-oriented in a(*)- and c(*)-axis directions, though disordered parts intervened among the crystallites. This is attributed to the fact that, though the content of PHB in the segments of disordered parts was only 50%, these PHB held the co-orientation among the slender crystallites within one group. Heat treatment induced the development of block segment and subsequent crystallite growth with fiber. This reorganization improved the thermostability and the mechanical properties. Copyright (C) 2000 John Wiley and Sons, Ltd.