The complex nature of, and importance of detritus consumption in, estuarine food webs make analysis of specific pathways of carbon in estuaries very difficult. We have evaluated the use of the natural abundances of the stable carbon isotopes in estuarine flora and fauna as a qualitative indicator of food sources in a Georgia salt marsh estuary. The salt marsh ecosystem is particularly amenable to this technique because of the wide span of distinctive 13C/12C ratios of the dominant plants °13C of ‐12 to ‐26%). Laboratory experiments showed negligible further fractionation of the characteristic °13C of marsh plants during microbial degradation of dead plant material, and during assimilation of ingested detritus by estuarine invertebrates. Invertebrates having a variety of feeding habits were collected in monospecific and mixed species stands of marsh plants. Marsh snails and insects had °13C values close to those of the associated vascular plants. Deposit—feeding crabs had isotope ratios biased away from the marsh plant °13C values, likely a result of feeding on benthic diatoms and out—of—stand foraging. Mud snails and filter—feeding bivalves showed 13C/12C ratios similar to those of benthic and pelagic algae. The °13C values of marsh fiddler crabs varied from ‐12 to ‐24% in various habitats having predominately C4 or C3 food source plants. Individuals collected from populations of 3 crab species in a Spartina alterniflora marsh had 13C/12C ratios with a standard deviation of only about 1% from the mean°13C. Stable carbon isotope ratio analysis appears to be a powerful tool in the evaluation of carbon pathways in estuarine food webs.