The six third‐order moduli of GaAs have been determined by measurement of ultrasonic wave velocities (time delays) as a function of applied stress. The adiabatic values (in units of 1012dyn/cm2) at 25°C and for zero electric field are very similar to those previously obtained for Si and Ge, as can be seen from the following table:ModulusGaAsGeSiC111−6.22±0.06−7.10−8.25C112−3.87±0.03−3.89−4.51C123−0.57±0.06−0.18−0.64C144+0.02±0.09−0.23+0.12C166−2.69±0.06−2.92−3.10C456−0.39±0.09−0.53−0.64.