The crystal structure of poly (2,6-naphthalenebenzobisthiazole) (Naph-2,6-PBT) was studied using X-ray and molecular modeling methods. The X-ray pattern of the annealed Naph-2,6-PBT fiber showed several Bragg reflections as well as streaks along the layer lines indicating that the registry between adjacent chains exists in the crystal with a great deal of axial disorder. Disordered structure in the crystal was fitted into the triclinic unit cell with the unit cell parameters of a = 6.78 angstrom, b = 3.46 angstrom, c = 14.61 angstrom, alpha = 88.0 degrees, beta = 114.7 degrees, and gamma = 94.8 degrees with PI space group. The calculated density, 1.68 g/cm(3) was comparable with the observed density, 1.56 g/cm(3). The Delta c/c (staggering ratio) representing the registry between the adjacent chains in the ac plane was - 0.19, which is in good agreement with the energy calculation although another local energy minimum was found at Delta c/c=0.31. The disordered structure in Naph-2,6-PBT was probably due to the discrete axial shift between Delta c/c = 0.31 and - 0.19 in the ac plane. The LALS refinement showed that the naphthalene group was rotated by 9 (+/-3)degrees from the ac plane on the projected structure along the chain axis with a torsion angle between the naphthalene and benzobisthiazole rings of 23 degrees. (c) 2005 Elsevier Ltd. All rights reserved.