Mangrove horseshoe crab Carcinoscorpius rotundicauda were tracked for 6 mo in the tropics using acoustic telemetry to determine whether they either show homing behaviour, remain near the site of capture, or travel to the open sea. Twelve adult horseshoe crabs were caught on the Mandai mudflats at Kranji and acoustic telemetry transmitters attached. Six acoustic receivers were deployed suspended from floating fish farm platforms ~1 km apart along the Strait of Johor, an estuary off the north coast of Singapore. Four individuals, 2 of each sex, were used in each of 3 tests: 4 crabs were released at the site of capture, 4 were released at a second sitẽ 5.7 km away to determine if they exhibited homing, and 4 were caught along the muddy beach at the second site, tagged and released into deep water under one of the receivers. Although there were gaps in detection, 11 of 12 crabs were located intermittently within ~6 km of the point of release. One crab disappeared after 5 d. Homing was not exhibited, and the crabs were detected within the estuary for 6 mo. We concluded that the crabs in the tropical estuary, where there are no seasons or marked changes in water temperature, showed no tendency to travel out to sea and did not exhibit homing behaviour or any synchronized movement pattern to deeper waters.