Temperature dependence of the elastic modulus El of the crystalline regions of poly(p-phenylene benzobisthiazole) (PBZT) in the direction parallel to the chain axis was investigated by X-ray diffraction over a temperature range from - 180 to 400 degrees C. The EI value of PBZT was 372 GPa at room temperature, which was the highest El value ever investigated. This value coincided with that reported by Lenhert and Adams (395 GPa) [Lenhert PG, Adams WW, Mat. Res. Sec. Symp. Proc. 1989; 134: 329] within experimental error. The axial chain thermal expansion coefficient was constant at - 2.9 x 10(-6) K-1 from 20 to 445 degrees C, and correspondingly the El value remained unchanged up to 400 degrees C. Moreover, the El value at - 185 degrees C also coincided with that at room temperature. It is considered that the deformation mechanism remains unchanged over a wide temperature range and the skeletal conformation of PBZT in the crystalline regions is stable against the heat.