A phase-shifting homodyne interferometer with feedback control by heterodyne phase detection is developed. The backreflected beam from a reference flat with a small wedge angle and an internal heterodyne detection system are utilized to obtain heterodyne beat signals for the feedback error signal. Using a digital high-speed lock-in amplifier, interference fringe stabilization, and accurate pi/2 phase shifts when micrometer-order vibrations are applied have been successfully demonstrated with a general nonpolarizing homodyne interferometer configuration.