The potential advantages of the Mach-Zehnder Heterodyne Interferometer as a rotation sensing device are discussed. Recently, a Mach-Zehnder Heterodyne Interferometer was described which allowed the direct reading of changes in the relative phase between the two beams. This was achieved by the use of a phase comparator that develops a voltage which is linear with increasing relative phase. The purpose of this note is to point out the potential advantages that such a device could have as a rotation sensor. Of course, the capability of a passive interferometer to sense rotations has been known since Sagnac's work. More recently, interest has centered on the ring laser gyro as an active interferometer. Both devices have disadvantages that limit their usefulness. The Sagnac Interferometer reflects rotational motion by a fringe shift which is proportional to the rate of rotation.