Acute and sublethal toxicity of surface sediments collected from five coastal and estuarine sites around and Irish coast were assessed in October 1997 using a whole sediment toxicity bioassay. Six hundred individuals of the estuarine bivalve, Tapes semidecussatus (1 + age class) were exposed to field-collected sediments for a period of 21 days. Survival, biometric, behavioral, biochemical, and histochemical measurements were taken before, during, and following exposure. Between 93 and 98% mortality was observed in clams exposed to sediments collected from Dublin and Bantry Bays, respectively. One hundred percent survival was observed in both reference and control sediments during the study. There was significant variation in burrowing behavior of the clams observed during the study. Glycogen levels in the soft tissue of all animals decreased during the bioassay. There was no association between metal (Zn, Pb, and Cu) concentrations in the dry flesh of clams exposed to sediments and sediment metal (Zn, Pb, and Cu) levels. Lipofuscin accumulation was observed in the digestive diverticula of clams exposed to all contaminated sediments with the exception of the reference and control sediments. The potential of T. semidecussatus as a bioassay species of estuarine sediments is discussed. (C) 2000 by John Wiley and Sons, Inc.