The effect of relative humidity on the fracture energy release rate, G(Ic), for single nylon 6,6 fibers has been determined previously. In this article, it is shown that G(Ic) is independent of relative humidity for moisture contents of > 2.3% once the plastic zone correction is made. G(Ic) is compared with various proposed mechanisms to account for fracture energy. It is shown that the energy required to disrupt or "melt" the crystals in the plastic zone accounts for the majority of the energy required to break the specimen, and should be considered explicitly in future analyses of fracture in semicrystalline polymers.