We determined the stable isotope ratios of carbon and nitrogen of three decapods, Ilyoplax pusilla, Macrophthalmus japonicus, and Helice tridens, that dominate the Tamagawa Estuary as well as those of other coexisting benthic animals, benthic diatoms, terrestrial plants, surface sediment, and suspended particulate organic matter present in the estuary water. The results suggest that I. pusilla feeds on benthic diatoms as its primary food source; H. tridens feeds on other benthic animals in addition to organic matter in the bottom sediment, while M. japonicus feeds on benthic diatoms and other animals. The values of total organic carbon (TOC), total nitrogen (TN), and stable isotope ratios of sediment samples from the burrow walls and mud around the burrow entrances of M. japonicus were similar to those of surface sediment samples. However, sediment samples from the burrow wall and mud balls of I. pusilla showed lower TOC, TN, and δ¹⁵N values than the surface sediment samples, which suggests that I. pusilla uses mud devoid of organic materials to construct the burrow wall.