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Large marine predators like sharks and whales can have a substantial influence on oceanic ecosystems, and characterizing their interactions with the physical and biological environment is an important goal in marine ecology. Studies of foraging ecology are of particular importance, but sampling prey aggregations encountered by these predators is extremely difficult because of the small spatial scales over which prey aggregations often occur (meters to hundreds of meters). We developed the real-time acoustic tracking system (RATS) to allow large marine predators to be accurately tracked over these small spatial scales to facilitate proximate environmental sampling. The system consists of an array of four free-floating buoys capable of detecting 36-kHz pings emitted by an animal-borne acoustic transmitter. Upon detection, the buoys transmit their position and the arrival time of the ping via a radio modem to a computer on board a nearby ship, and a software program uses differences in arrival times from all of the buoys to estimate the location of the tagged animal. The positions of the tagged animal, buoys, ship, and support boats can be monitored via a graphical user interface to allow proximate environmental sampling and maintenance of the array around the tagged animal. In situ tests indicate that average positional accuracies for a transmitter inside either a four- or three-buoy array (buoys spaced 1-1.75 km apart) are less than 10 m, and that accuracies remain near 10 m for transmitters located up to 500 m away from the edge of the array. The buoys can consistently detect the transmitter up to 1000 m away, but detection rates decrease between 1000 and 2000 m; no detections were obtained beyond 2300 m. Field deployments of the system have demonstrated an unprecedented ability to monitor the movements of baleen whales in real time, allowing a suite of prey and oceanographic observations to be collected within meters to tens of meters of a tagged anim- - al.