The light diffraction phenomenon from Kevlar 49 fibers exhibits unusual polarization effects and its angular and wavelength dependence cannot be described by the grating equation. This unexpected behavior is analyzed by modeling the fiber as an anisotropic grating of finite thickness in which the diffraction arises from a periodic change in the orientation of the molecular dipoles. Theoretical predictions on this basis explain all available experimental observations. These findings are discussed in regard to currently proposed structural models for the fiber.