Over 380 stable carbon isotope (δ13C) analyses made during 1981–82 showed that Syringodium filiforme Kutz seagrass meadows in the Indian River lagoon of eastern Florida have food webs based on algal rather than seagrass carbon. Seagrasses averaging approximately-8‰ were isotopically distinct from algae epiphytic on seagrass blades (X=-19.3‰) and particulate organic matter in the water column X=-21.6‰. δ13C values of most fauna ranged between-16 and-22‰, as would be expected if food web carbon were derived solely from algal sources. These results counter the idea that seagrass detritus is the dominant carbon source in seagrass ecosystems. Two factors that may contribute to the low apparent importance of seagrass in the study area are high algal productivities that equal or exceed S. filiforme productivity and the high rates of seagrass leaf export from meadows.