An ultrasonic interferometer technique for measuring the temperature dependence of the relative change in elastic constants is described in detail. Results are reported for measurements carried out as a function of frequency from 10 mc/sec to 170 mc/sec, for compressional wave propagation along the [100] axis of silicon over a temperature range of −60° to +15°C.The mean value of (1/c11)(dc11/dT) obtained is 55.59±0.81×10−6(°C)−1, and no frequency dependence of (1/c11)(dc11/dT) was found.