Stable isotopes of sulfur and carbon were used to trace the dominant flows of organic matter from producers to macroconsumers in Great Sippewissett Salt Marsh on Cape Cod. Spartina alterniflora and sulfur-oxidizing bacteria assimilate isotopically light sulfides produced via sulfate reduction, and this light sulfur was detected in consumers. In contrast, phytoplankton and upland plants assimilate isotopically heavier SO4 2- with little or no fractionation. A dual-isotope approach using both delta 13C and delta 14S showed that Ilyanassa obsoleta and Fundulus heteroclitus depend very heavily on Spartina detritus, while filter feeders such as Crassostrea virginica and Geukensia demissa depend on a mixture of plankton and Spartina detritus. Spartina detritus and plankton were both much more important as organic matter derived from terrestrial inputs.