The stable carbon and nitrogen isotope ratios of mangrove leaves, particulate organic matter (POM), shrimps and crabs, squids and fishes were studied for tracing the actual food web structure in the Matang mangrove brackish water ecosystems. Carbon isotope ratio of POM showed a positive correlation with the level of salinity, suggesting that POM around the mouth to mudflat areas of the river may consist of phytoplankton-derived detritus from the sea, whereas POM from mid- to upstream areas of the river may consist of mangrove-derived detritus. δ¹³C-δ¹⁵N map showed a C-N isotope ratio gradient with the lowest value with mean (±SD) of δ¹³C (-28.74±0.031‰) and δ¹⁵N (4.36±0.15‰) for a mangrove leaf (Rhizophora apiculata), the highest value of δ¹³C (-20.45±0.026‰) for a squid (Loligo duvaucelii) and of δ¹⁵N (13.63±0.021‰) for a fish (Stolephorus commersonii). Penaeid shrimps (Penaeus merguiensis, Metapenaeus brevicornis and M. ensis) and mangrove crabs (Scylla serrata) showed intermediate C-N values between those of mangrove leaves and fish/squid.