Adult oysters, Crassostrea ~r~nica (Gmelin) were held in seawater containing 10 or 1OO ppb mercury in the form of mercuric acetate for 45 days. Mercury concentration in tissues was determined by analysis of individually homogenized oyster meats using wet digestion and flameless absorption spectrophotometry. After 45 days, average mercury tissue concentration was 91,600 and 12,1OO ppb in the 1OO and 10 ppb mercury groups, respectively. A slight decline in mercury residues in the 1OO ppb group during the accumulation period was attributed to spawning. Clearance of mercury from tissues was studied in a constant temperature regime (25oc โข 2CO) for 25 days and in a declining temperature regime (25 ~ to 5oc) for 80 days by exposing treated adults to estuarine water with no mercury added. The biological half-life of mercuric acetate was 16.8 and 9.3 days in the 25oc temperature regime, and 35.4 and 19.9 days in the declining temperature regime, for the 10 and 100 ppb groups, respectively. Smaller oysters (O to 7 g) consistently accumulated more mercury per gram wet weight than larger oysters (7 to 20 g) in populations exposed to 10 and 100 ppb mercury. introduction Estuarine and coastal waters increasingly have become repositories for the effluents of both industrial and agricultural activity. Mercury concentrations in coastal marine organisms reflect increased concentrations in seawater (Klein and Goldberg, 1970). Many molluscan species inhabit estuaries and, since the sessile adults are unable to migrate to less polluted areas, they must adjust to increasing concentrations of mercury in seawater or die. Several authors have documented mercury accumulation and deputation in bivalves: venus japonica (Irukayama, 1962) ; Mercenaria mercenaria and Spisula sp.