The equations of motion for an elastic nonisotropic solid are reduced to a form useful for determination of third‐order elastic constants by means of ultrasonic pulse distortion measurements. Values of the coefficients in the reduced equations of motion are tabulated for two cubic materials. The tables are used to show that for cubic materials one should be able to measureC111,C112, andC166with reasonable accuracy. An argument is given which shows that the equations of motion for a single plane wave in a cubic crystal depend on the five parametersC111,C112,C166, (2C144+C123), and (½C144+C456) instead of all six third‐order elastic constants.