The tensile properties of poly (paraphenylene benzobisoxazole) or PBO fiber strands were studied using two variables: gage length and the number of twists per inch. The gage length was varied from 1 to 10 inches with 2 twists of the fiber per inch. The effect of the number of twists per inch was studied by varying the number of twists from zero to 10 along a 5-inch gage length. The trends of tensile strength and modulus due to these variables were established and appropriate explanations of these behaviors are provided.The coefficient of thermal expansion (CTE) was studied on bare strands of PBO and Kevlar 49 (a product of Du Pont de Nemours & Co.) fibers, using a Du Pont 943 Thermomechanical Analyzer (TMA) equipped with a film and fiber tension assembly. The axial CTE of both fibers exhibited a dependence on the small dead load employed to keep the fibers straight. Kevlar 49 fiber, when wet, attained a much less negative value of CTE than when dry. In contrast, PBO fiber absorbed very little moisture, and the CTE remained unchanged.