A study was undertaken to determine the efficiency with which a marine deposit-feeding clam (Macoma nasuta) extracted hexachlorobenzene (HCB) from water (EPW). An exposure chamber (clambox) was designed that separated the inhalant and exhalant siphons, allowing the collection of ventilated water. Seawater dosed with (14)C-labeled HCB was pumped into the inhalant chamber of the clambox. Clams were exposed to three temperatures (12, 17, 22C) to vary weight-specific ventilation volume (Vg). Loss of HCB from the exhalant chamber precluded determination of EPW from the difference in HCB concentrations between the inhalant and exhalant chambers. Instead, gross EPW was calculated by dividing the HCB tissue residues by the amount of HCB to which the clam was exposed (water ventilated x the HCB concentration). Gross EPW averaged 82%. Correcting for non-gill uptake (surface sorption of HCB),gill EPW averaged 64-66%, and did not decrease with increasing Vg. In M. nasuta, Vg varied less than two-fold, which may explain the lack of a ventilation effect on EPW. HCB tissueresidues were lineraly related (R (2) = 0.93) to gill exposure. The liner relationship between tissue residues and exposure supports a bioenergetics-based bioaccumulation model andindicates that factors that increase Vg, such as low oxygen concentrations, would result in more rapid uptake and a greater body burden. (Copyright (c) 1988 Elsevier Science Publishers.)