A self-contained, bottom-mounted instrument for measuring high-resolution vertical profiles of horizontal current in the deep ocean over long time periods (up to a year) has been designed, constructed and successfully tested. It employs an interferometric technique to track small, expendable, buoyant probes containing continuous wave (CW) acoustic beacons which are released on a preset schedule. A carrying capacity of 60 probes is quite feasible. Tracking data are stored on cassette tape and recovered with the bottom instrument. The design of the instrument is discussed and errors analyzed. The uncertainty in the horizontal current averaged over 10 m vertical intervals increases near the surface to approximately 2 cm s−1 and is due primarily to acoustic phase noise from reflection off the rough sea surface. Considerable improvement in future design can be expected.