The nature of anisotropic fluctuation modes in an ordered polymeric system is analysed using general symmetry arguments. It is shown that the anisotropic fluctuation modes in a periodic phase can be classified using a wave vector within the irreducible Brillouin zone and a band index. The spatial profiles of the fluctuation modes are described by Bloch functions which are plane waves modulated by periodic functions. These general statements enable the study of the stability and kinetic pathway of complex ordered polymeric structures.