Instruments for the recording of physical variables such as tides, currents and temperatures have a long history in oceanography [Sverdrup et al. (9)]. A more recent development is the use of autonomous instrument platforms, known as landers, that descend to the deep sea floor to monitor biological activity [Tengberg er al. 10)]. [Priede et al. (8)] showed that miniature acoustic transmitters imbedded in baits could be tracked using a lander-mounted receiver system. This provided a means of directly measuring the rate of dispersal and transport by scavenging fishes of material dumped on the sea floor. This paper describes a new lander vehicle (AUDOS III, Aberdeen University Deep Ocean System) equipped with a camera, current meter and a 77kHz active sonar with three receiving hydrophones spaced on a triangular array orientated in the horizontal plane. Fish attracted to bait placed beneath the array are photographed and those that ingest code activated transponders (CATs) placed in baits are subsequently tracked across the sea floor. A 10 kHz acoustic telemetry system provides a communications and command link to the surface ship. The system is capable of operating to a depth of 6000m. The principles of the tracking algorithm and its implementation within the architecture of the vehicle are described.