The nonlinear evolution of electrostatic plasma waves excited by mode conversion is studied in an inhomogeneous, collisionless, unmagnetized plasma. Experiments in the microwave range (f = 3.5 GHz), performed in a plasma (ne ≃ 1011 cm−3) created in a multipolar discharge, show that the electrostatic plasma-wave evolution exhibits a transition from a nonlinear steady-state regime toward chaos, occurring when the pump field or the gradient length is increased. The Zakharov equations, which model the plasmawave evolution coupled with low-density perturbation, are solved numerically with parameters close to those of the experiment; this simulation allows a characterization of the two regimes.