ABSTRACT: The Gulf of Mexico supports dense aggregations of megafauna associated with hydrocarbon seeps on the Louisiana Slope. The visually dominant megafauna at the seeps—mussels and tube worms—derive their nutrition from symbioticrelationships with sulfide or methane-oxidizing bacteria. The structure of the tube worm aggregations provide biogenic habitat for numerous species of heterotrophic animals. Carbon, nitrogen and sulfur stable isotope analyses of heterotrophic faunacollected with tube worm aggregations in the Green Canyon Lease area (GC 185) indicate that most of these species derive the bulk of their nutrition from chemoautolithotrophic sources. The isotope analyses also indicate that although 2 species may bederiving significant nutritional input from the bivalves, none of the species analyzed were feeding directly on the tube worms. Grazing gastropods and deposit-feeding sipunculids were used to estimate the isotopic value of the free-livingchemoautolithotrophic bacteria associated with the tube worms (δ¹³C –32 to –20‰; δ¹⁵N 0 to 7‰; δ³⁴S –14 to –1‰). The use of tissue δ³⁴S analyses inconjunction with tissue δ¹³C and δ¹⁵N led to several insights into the trophic biology of the communities that would not have been evident from tissue stable C and N analyses alone.