Stable carbon isotope ratios (d13C) of sediments, suspended particulate matter, and crab gut content from Maruhubi mangrove forest, Zanzibar, were determined. Stable carbon isotope ratios were used to determine sources of organic matter in the forest. Mangrove organic carbon (d13C±SD = - 27.97±0.25) was found to be an important component of the sediment organic matter except at the marine fringe zone (d13C±SD = - 17.26±0.83). The areal distribution of d13C values show that the marine fringe zone was rich in organic carbon of marine origin. The terrestrial fringe zone was dominated by a mixed source of sediment organic carbon (d13C±SD =-24.89± 0.72). Vertical variations in d13C were not so obvious, showing a diminishing importance of mangrove organic matter below 60 cm in the sediments of the terrestrial fringe zone. Stable carbon isotope ratios of the major primary producers in the mangrove forest were reflected to a reasonable extent in the terrestrial fringe surficial sediment organic carbon. Stable carbon isotope ratios for plant and sediment organic carbon gave values in the range of - 27.67 to - 28.31 o/oo for mangroves and - 17.57 to - 24.89 o/oo for surface sediment (0 - 5 cm). Marine fringe sediments receive organic matter of marine origin to a large extent, in comparison to terrestrial fringe sediments, which contain mainly organic matter of mangrove origin.