An acoustic interferometer of improved design and the associated electronic circuit for measurement of ultrasonic velocities in liquids are described. A beat frequency oscillator is used to generate the ultrasonic waves in the liquid, and a vacuum‐tube voltmeter is employed as a detector of nodal positions. In order to secure adequate stability an electronically stabilized power supply is used. Measurements of velocities in acetone, benzene, and distilled water are compared with earlier workers and new measurements for heavy water at 5°, 10°, and 15°C are tabulated.