The effect of bottom sediment composition on the pelecypod mollusc,Rangia cuneata, was investigated in the Pamlico River estuary, North Carolina. Mortality, meat weight: shell weight ratio, and shell growth were evaluated in relation to particle size, organic matter concentration, and phosphate concentration in the sediment.In situ factorial experiments were carried out for 5 months using indivdually marked clams placed in boxes of predetermined sediment composition. Clay-silt sediments were found to be an unfavorable environmental substrate. The detrimental effect was greater in sediments containing high levels of organic matter and phosphate, suggesting that physical and chemical factors associated with finer sediments were responsible. However, in sand sediments, high levels of organic matter and phosphate were found to be favorable toRangia. To clarify these field results, clams were placed in trays with sediment containing zinc-65 labeled detrital matter and phosphate-32. The results indicated thatRangia can accumulate these materials from the sediment. AlthoughRangia is morphologically a typical filter-feeding bivalve, this species appears to obtain organic matter and phosphate from the sediment by direct ingestion or by feeding on bacteria associated with these materials. Such mechanisms are suggested as a possible cause of the better growth observed in sand sediments with moderately high levels of organic matter and phosphate.