An experiment on large amplitude electrostatic waves is described, the results of which illustrate the phase velocity variations during the first bounce period predicted by the single‐wave model. The spatial growth of sidebands, after the initial large amplitude wave has reached saturation due to nonlinear trapping effects, is also described. It is found that the amplitude of the phase velocity variation is proportional to&phgr;1/2(where&phgr;is the wave amplitude) and that the slower phase velocity, higher‐frequency sideband (for instability near the plasma frequency) is always the dominant one.