Experiments were performed on the modulation of CO2laser radiation (10.6 &mgr;) at the piezoelectric resonance frequencies of a GaAs sample. The GaAs crystal length was 5.09 cm in the[11¯0]direction with a cross section of 0.291 cm [110] × 0.278 cm [001]. Experimentally, one shear and two thickness vibration modes were observed. The voltages to yield half‐wave retardation by means of the piezoelectro‐optic effect were found to be 4.15 V at the shear resonance frequency (736.900 kHz) and 15.4 and 51.0 V at the first (675.800 kHz) and second (1019.130 kHz) thickness resonance frequencies, respectively. The bandwidths were in the range 30–100 Hz. When the electro‐optic modulation scheme is used instead of the resonant piezoelectro‐optic modulation scheme, 5.2 kV are required to obtain half‐wave retardation with the same sample. The shape of the sample precluded analytically solvable models for the observed phenomena. However, approximate models yield reasonable order‐of‐magnitude agreement with the observed values.