Microwave nonlinear effects have been observed in the plasma sheath surrounding a probe. Results were obtained for a PIG discharge operating at 2.0×10−2Torr xenon. The nonlinear effects include mixing, harmonic generation, and current rectification. The probe is connected directly to a 50‐&OHgr; coaxial transmission line through which the microwave signal is coupled to the sheath. Strong nonlinear interaction is observed when the probe is dc biased at a point in the exponential region of the Langmuir curve. The explanation of this interaction is based on the usual interpretation of the sheath as a nonlinear capacitance.