The effect of in situ heating on the unit cell dimensions of crystalline aramid fiber (Twaron((R)) fibers) has been studied in the temperature range 2G-280 degrees C. The changes in the unit cell dimensions have been used to estimate the coefficients of thermal expansion along the axial directions. On heating, the a and b axes expand, the coefficients of thermal expansion being alpha(a) = 35.84 x 10(-6)/degrees C and ab = 38 X 10(-6)/degrees C at room temperature. The c-axis contracts on heating, giving negative coefficient of thermal expansion alpha(c) = -5.38 x 10(-6)/degrees C at room temperature. Volume coefficient of thermal expansion alpha(v) however is positive, being 101.22 x 10(-6)/degrees C. POLYM. ENG. SCI., 48:211-215, 2008. (c) 2007 Society of Plastics Engineers.