The effect of relative humidity on the fracture of single nylon 6,6 fibers is determined using fracture mechanics. The fracture energy release rate, G(Ic), of these fibers is shown to be 8 X that for injection-molded nylon 6,6 at 0% RH. G(Ic) varies from 31.3 kJ/m2 at 0% RH to 15.6 kJ/m2 at 100% RH. The dependence of G(Ic) on RH is in agreement with previous studies indicating two types of water in nylon : a tightly bound and a loosely bound type. In addition, G(Ic) is shown to be 10 X greater for transverse fiber breaks than for axial splitting of the fiber due to the high degree of orientation in these fibers. These studies are the first of their kind for organic fibers. 1994 John Wiley & Sons, Inc.