The Code Activated Transponder (CAT) ingestible fish tag capable of operating at depths of 6 000 m is described. The CAT is acoustically interrogated by a scanning sonar mounted on the AUDOS free-fall camera lander vehicle. The CAT, 12.5 mm diameter and 65 mm long works at ultrasonic frequencies (77 KHz) and can be tracked to a range of 500 m. The AUDOS lander incorporates the new TRATEX (Transponding Acoustic Tracking Experiment) system that logs direction and range of individually tagged fish together with current and compass information. Typical results are shown from a track of 3 Coryphaenoides (Nematonurus) armatus (Macrouridae) taken at the Porcupine Seabight in the North Atlantic (49° 35'N-13° 43'W), 3 500 m deep. The mean fish swimming speed was 0.087 m.s−1. Despite their low speed, fish moved independently of the bottom current, either cross-current or against the current, optimising the chance of encountering odour plumes from new food falls.