In traditional interference lithography, interference fringes are typically phase locked to a stationary substrate using analog homodyne photodiode signals that are fed back to control a phase-shifting device such as an electro-optic modulator or a piezoelectrically transduced mirror. Commercially available fringe-locking systems based on this approach often achieve stability of the interference fringes to within a small fraction of the fringe period p (typically +/- p/20 peak-to-peak). We describe the performance of a heterodyne fringe control system utilizing acousto-optic phase shifters and digital controls that is designed to satisfy the much more stringent fringe control requirements for scanning beam interference lithography. We demonstrate locking to +/-p/100, and expect further significant improvements. This versatile system can also be used to lock the phase of moving fringes in almost arbitrary fashion at fringe velocities up to 2.5 x 10(7) periods/s and to measure the phase of gratings.