The carbon and nitrogen stable isotope ratios of the mangrove sesarmid crabs Parasesarma erythrodactyla, Paragrapsus laevis, and Helograpsus haswellianus were measured from sites in arid and humid temperate Australia, and compared with potential food sources. Crab stable isotope ratios were uniform and consistent with published values from subtropical Queensland. Fresh and decomposed leaves of the dominant mangrove Avicennia marina were discounted as a potential food source, being too depleted in their δ13Cand δ15Nsignatures compared to crabs to have made a substantial contribution to the diet. A three-source mixing model indicated that fine benthic organic material was the primary food source. Mangrove epiphyte and highly decomposed mangrove leaf material were too depleted in δ15 N to be contributing to crab diet.