The dispersion of the sidebands of a large amplitude plasma wave has been measured. Sidebands propagate according to a beam‐plasma type dispersion. Sideband dispersion is calculated with a quasi‐linear theory using the locally measured time‐averaged electron velocity distribution. This model assumes that the large wave and the sidebands interact primarily through the time average of the perturbation the large wave causes in the velocity distribution. The measured sideband dispersion agrees with the predictions of this model.