Many marine infaunal animals form organic tube and burrow linings. The role of these materials in organic matter cycling and preservation in sediments is largely unknown. In the case examined here, the infaunal sea anemone, Ceriantheopsis americanus, (a common component of bottom communities along the east coast of North America) forms a leathery, fibrous tube lining 2–3 mm thick, ∼1 cm in diameter, and typically extending 20–30 cm into deposits. Tube fibers (∼2 mm long, 2–5 μm thick) formed from discharged specialized nematocyst cells, ptychocysts, are composed of a silk-like protein copolymer, cerianthin. Tubes incubated under oxic and anoxic conditions over a period of 122 days demonstrate that initial rates of whole tube decay are 10–100 times slower than usually found for fresh planktonic debris and aquatic macrophytes despite a relatively low molar C:N ratio of ∼5.1. First order decomposition rate constants in oxic water, anoxic water and anoxic sediment are ∼0.76, ∼0.41 and ∼0.22 yr–1 for particulate tube carbon and ∼0.2, ∼0.1 and ∼0.1 yr–1 for particulate nitrogen, respectively (20°C). There are no obvious (under SEM) morphological changes in tube fibers during initial tube decomposition, implying slower long term rates. Although slow, tube decomposition stimulates bacterial activity in sediments from below ∼10 cm depth where any organic matter present is even more refractory than the tubes themselves. In central Long Island Sound muds, tubes apparenlly account for a minimum of ∼0.6–1.8% and 2.8–8.4% of the steady state C and N detrital pools in the upper 10–30 cm of the sediment. C. americanus tube production apparently accounts for ∼9% of the average particulate carbon and ∼12% of the nitrogen fluxes to the benthos. Tube construction by infaunal benthos may thus represent an important pathway for refractory compound formation and organic matter preservation.