Intertidal grazing crabs play an important role in estuarine ecosystems, transforming carbon fixed by autotrophs into forms available to a wide range of consumers. Whether the autotrophic carbon is derived primarily from intertidal vegetation or microalgae is an important question to be resolved, as the modification of estuaries alters the balance between these potential food sources, and restoration efforts are best guided by an understanding of the primary drivers of ecosystem energy flow. We utilised the mosaic of C 3 and C4 vegetated patches in a temperate saltmarsh to clarify the relative contributions of potential sources of carbon and nitrogen to the diet of 2 species of grapsid crabs: Paragrapsus laevis and Helograpsis haswellianus. The 2 vegetation communities occupied the same position in relation to tidal elevation. We analysed stable isotopes of carbon (δ13C) and nitrogen (δ15N) to discriminate 3 potential sources of dietary carbon using an IsoSource mixing model: microphytobenthos (MPB); fine benthic organic matter (FBOM); and fresh plant material. We found enrichment of δ13C and depletion of δ15N in crabs sampled from patches of the C4 grass Sporobolus virginicus, consistent with the use of C4 derived carbon compared to those sampled in the C3 chenopod Sarcocornia quinqueflora. However, microphytobenthos was similarly depleted within large patches of S. virginicus, implying uptake of dissolved inorganic carbon originating from plant respiration. Multiple-source mixing (IsoSource) models indicated a primary role for MPB and FBOM in crab diets, with locally derived plant material making little contribution to crab diet. The result contrasts with those of studies from subtropical and tropical systems.