Polycondensation in polyphosphoric acid of 2,5-diamino-1,4-benzenedithiol dihydrochloride with 2-(2,6-dimethylphenoxy)terephthaloyl chloride led to rigid-rod benzobisthiazole polymers with thermally reactive dimethylphenoxy pendants. Soluble polymers with intrinsic viscosities as high as 15.5 dl/g (methanesulphonic acid, 30 degrees C) were obtained. The polymers became insoluble after heat-treatment at approximately 400 degrees C, possibly indicative of a crosslinking reaction between the polymer chains at the pendant sites. Fibres spun from a lyotropic polyphosphoric acid solution and heat treated at 425 degrees C exhibited a tensile modulus of 91 GPa, a tensile strength of 1.10 GPa, and a compressive strength of 400 MPa. Wide-angle X-ray scattering patterns of the polymer fibres indicated that the bulky pendants significantly enhanced the axial registration between the neighbouring rigid-rod polymer backbones.