The behavior of sidebands growing in the presence of a launched large amplitude wave in a beam–plasma system is studied numerically. The large amplitude wave traps the beam and both the upper and the lower sidebands become unstable. The behavior of the instability is found to be dependent on a phase relationship that exists between the two sidebands and the large amplitude wave. The essential features of the instability are well described by a simple model in which the trapped electrons are replaced by a single particle oscillating in each well of the large amplitude wave.