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The preferred habitats, home range and activity patterns of sub-adult mulloway Argyrosomus japonicus (Sciaenidae) in the Georges River, New South Wales, Australia, were investigated using ultrasonic telemetry. Tags were surgically implanted in 9 hatchery-reared and 12 wild-caught mulloway (330 to 730 mm total length, TL). Fish were tracked for 2 periods of continuous tracking over 72 h in a 15 km section of river, once daily for a 20 d period, and up to 3 times mo–1 for 11 mo. Key habitats were identified as discrete holes or basins up to 20 m deep. Mulloway preferred this deep hole habitat as small fish (hatchery-reared, 300 to 500 mm TL) remained in these deep holes both day and night, while large fish (wild, 500 to 800 mm TL) ventured outside the holes at night. Maximum home range of small and large mulloway was 6000 and 17710 m2, respectively, and home range correlated significantly with length. Small fish moved up to 7 km d-1 while large fish moved up to 16 km d-1. Small fish released in shallow water initially had significantly greater movements than those released directly over deep holes, with movement up to 10 km in 3 d. Activity patterns varied between small and large fish, with significantly larger movements by large fish during the night and early morning than daytime. Five wild-caught mulloway tracked over 11 mo showed strong fidelity to holes within their particular home range. Mulloway should be stocked directly into their deep holes to minimise movements. The use of key habitats by mulloway indicate that their survival will be sensitive to stocking density. Optimal stocking density could be estimated from the area of key habitat in the target estuary.