Deposit-feeding invertebrates select particles for ingestion that are richer in organic carbon, and thus higher in pollutant concentration, than the mean values of bedded sediment. To estimate the ingested pollutant dose, a method was developed that uses the enrichment of the total organic carbon concentration (TOC) of the feces compared to the TOC of the bedded sediment as a measure of pollutant enrichment in the ingested particles. The deposit-feeding clam Macoma nasuta (Conrad) was exposed for 7 d in clamboxes to either bulk or fine sediment fractions that varied in TOC. Feces were collected and the TOC of sediment and feces estimated. The TOC of the feces was corrected for the carbon lost during digestion. The selectivity index (SI), the ratio between the corrected TOC of the feces and TOC of the bedded sediment, was determined to be 4.36 and 1.72 for the bulk and fine sediment fractions, respectively. The SI was then applied to data from a companion study in which M. nasuta was exposed to hexachlorobenzene (HCB) dosed sediment. The efficiency of gut uptake of HCB sorbed on ingested sediment ranged from 38 to 56%, with the lower value the more reliable. The effects of selection by deposit feeders for organically rich, high pollutant particles needs to be considered in experiments measuring the bioavailability of sediment-sorbed pollutants or in experiments attempting to determine the routes of pollutant uptake.