The mechanical hysteresis of rubber has been measured by almost every conceivable method, many of them simple but inexact. During recent years, however, the practical importance of hysteresis in rubber has been gaining recognition, and consequently a number of more elaborate and relatively exact methods and devices for measuring hysteresis have been developed. The present article describes a torsional hysteresis test. Although the apparatus required is quite simple, the test is accurate and may be regarded as a satisfactory test for many purposes.