A novel polyimide-based heterocyclic polymer, polyimide benzobisthiazole (PIBT), was prepared from diaminophenylene benzobisthiazole (DAPBT) and pyromellitic acid dianhydride (PMDA). Fibers with tensile modulus of 220 GPa were obtained from PIBT; this modulus value is consistent with the 430 GPa modulus determined by wide-angle x-ray diffraction (WAXD) analysis. Unoriented isotropic films exhibited significantly lower modulus values in the range of 10 GPa. Block copolymers of PIBT with thermoplastic segments were synthesized in an effort to prepare molecular composites. These materials showed a dramatic decrease in melt viscosity and had modulus values lower than observed in isotropic PIBT films.