Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were determined for the spotted scat (Scatophagus argus) in the Matang Mangrove Forest Reserve in Peninsular Malaysia. The ratios from fish samples along with those obtained from samples of their potential food items (i.e., mangrove leaves in the estuary, coastal phytoplankton, microphytobenthos) revealed wide variations, indicating that S. argus utilizes various food items between the coastal area at the river mouth to the inner mangrove estuary. The carbon isotope ratio in the fish body tended to be lower in fish sampled from the inner estuary as compared with fish at the river mouth. The carbon isotope ratio values of the fish body were relatively high in small fish (< 20 mm in total length [TL]) but lower in fish of 20–100 mm TL, and relatively higher in fish > 100 mm TL. These findings strongly suggest that small-sized S. argus (< 20 mm TL) migrate from the outer coastal area into the mangrove estuary and utilize the mangrove estuary habitat as their nursery grounds before eventually returning to the river-mouth area as their growth progresses.