Resonant absorption of an electromagnetic wave (f=3.5 GHz) in a multipolar discharge (ne≂1011 cm-3) with an adjustable density gradient is studied. Around the resonant location, nonlinear effects appear, associated with the ponderomotive force arising from the plasma wave. These effects are modelled by the Zakharov equations. Experimental and numerical results both exhibit a transition from a nonlinear steady state regime toward chaos when the amplitude of the electromagnetic wave or the gradient length is increased. ¿American Geophysical Union 1991