In this study we investigated potential carbon sources for the capitellid polychaete, Heteromastus filiformis. It is a head-down deposit feeder ingesting sediment from at least 15 cm below the sediment-water interface. This orientation appears to minimize the worm's ability to acquire food and oxygen and maximize its exposure to sulfide. The food sources we examined were metabolically active bacteria, benthic algae, detritus and chemoautotrophic bacteria. Carbon retention efficiencies from metabolically active bacteria, benthic algae and detritus by H. filiformis were 26%, 8% and 4% respectively. These values are relatively low compared to other deposit feeding species suggesting that H. filiformis does not possess unique digestive capabilities. Rubisco (Ribulose bisphosphate carboxylase) assays were negative, which indicates an absence of symbiotic chemoautotrophic bacteria in tissue or absorbed carbon. Average δ 13C were −12.83 for worms and −20.70 for 15 cm sediment, which indicates that external gardening of chemoautotrophs is not a major carbon source for H. filiformis. Nevertheless, several experiments showed that this capitellid worm had an unusually high gross heterotrophic CO2 uptake. We suggest that H. filiformis utilizes both dissolved and particulate carbon sources stored within anoxic and sulfidic sediments that are not utilized by other deposit feeding organisms.