A technique is described for the automatic measurement of ultrasonic velocity changes of one part in 105in continuous wave experiments. An oscillator is frequency modulated by an analog circuit which terminates frequency change when a mechanical resonance is achieved in an ultrasonic interferometer. Velocity changes appear as changes in terminal frequencies of the oscillator and are easily recorded. A typical result is presented. The possibilities of using the technique to measure temperature changes of 10−3°C and displacements as small as 10−6cm are discussed.