SYNOPSISHeat treated poly(p-phenylene benzobisthiazole) (PBZT) fibers tested in tension result in two types of failure modes. In failure mode I, the fiber exhibits a relatively sharp break; mode I1 is characterized by significant axial fiber splitting. Approximately 20% of the fibers failed in mode I1 when tested at 2.54,7.62, and 12.7 cm gage lengths. At 1.25 cm gage length all the fibers failed in mode I. Tensile strength decreased from the 1.25 to 7.62 cm gage length, but tensile strength of the fibers tested at 7.62 and 12.7 cm gage lengths were similar. The two failure mode observation is supported by the Weibull statistical distribution. Fiber tensile properties were also measured at 150°C. Axial compressive strength of the PBZT fibers as determined from the recoil test is also reported and is found to be independent of fiber tensile modulus and fiber diameter. As-received heat-treated fibers were post heat treated between 700 and 775°C. Structural changes in the fiber were studied using infrared spectroscopy, small and wide angle x-ray scattering, and swelling studies. 0