A model which treats the formation of wave sidebands on a propagating electron plasma wave in the case where the plasma density is modulated by a low‐frequency perturbation is developed. A new mechanism for production of an asymmetric distribution of sidebands appears, and circumstances under which the original wave may reconstitute itself are described. An experimental study of electrostatic plasma waves in a weakly ionized plasma, where the density is modulated either by external means or by the presence of an unstable drift wave, is in agreement with the model.