The nonadiabatic behavior of electrostatic oscillations in an initially unstable, collisionless, field‐free plasma is discussed. The equations of ordinary quasilinear theory are generalized to include the time rates of change of the oscillation frequency and the spatially homogeneous part of the distribution function. Certain aspects of the more general theory are illustrated by a discussion of the problem of interpenetrating electron and ion beams. The behavior of the instability is found to differ markedly from that predicted by quasilinear theory. Many of the findings are verified by a direct comparison with the results obtained by Buneman from computer experiments.