X-ray fiber diffraction and DSC investigations of the title polymer, P6BT, reveal the existence of three polymorphs. Both alpha- and beta-forms belong to the monoclinic system, while in the gamma-form, the chains are packed in a triclinic unit cell. In all three forms the bithiophenedicarboxylate group is confined in a pi-conjugated plane; however, with different orientations of the carbonyl group relative to the sulfur atom (in alpha- and beta-forms, S-C-C=O is in the cis conformation, while in the gamma-form, S-C-C=O is trans). The methylenic sequence in the beta- and gamma-forms is in the all-trans conformation with only slight deviations from planarity. With a torsion (tau(3)) around the C-O bond between the aromatic plane and aliphatic segment, the polymer chain changes its overall shape from ribbonlike in the alpha-form (tau(3) approximate to 180 degrees) to a sine-curve shape in the beta-form (tau(3) = 173 degrees) and finally to a zigzag in the gamma-form (tau(3) = 103 degrees). The alpha-form is paracrystalline, with random displacements of the chains along the fiber axis. The four chains in the unit cell of the beta-form are found in two pairs displaced by 5.63 Angstrom from each other, in the c-axis direction. Within the pair, the chains adopt an antiparallel disposition. The aromatic planes are arranged nearly parallel to the ac plane in a "side-by-side" pattern in the alpha-direction. Chains in the gamma-form pack in successive layers, where the bithiophene dicarboxylic planes aggregate in a "face-to-face" fashion, forming well-organized pi-stacks favorable for the interchain charge transfer of the alternating conjugated and nonconjugated polymer. The interplanar distance between neighboring bithiophene planes is 3.56 Angstrom The chain conformation and crystal packing in the gamma-form resemble those observed in the model compound, di-n-hexyl 2,2＇-bithiophene-5,5＇-dicarboxylate (6BT6). A monotropic mesophase with unique chain arrangements was found to occur during heating. The transformation temperatures are as follows : alpha- to beta-form, 130-150 degrees C; beta- to gamma-form, 161 degrees C; gamma-form to mesophase, 172 degrees C; and mesophase to isotropic Liquid, around 185 degrees C. The strong pi-pi interaction between aromatic planes is believed to be the main driving force of the phase transformations. Due to this solid-state ordering process, the bathochromic shift in the UV-visible region was observed. Some abnormal phenomena such as preferred tilt of crystal in the gamma-form are described. The relationship between these structures and those of the biphenyl analogue, poly(hexamethylene 4,4＇-biphenyldicarboxylate) are discussed.