The hard clam is an economically important bivalve and is abundant along the East Coast of the US. The goal of this research was to evaluate the sensitivity of this test species as compared to that of other benthic and epibenthic organisms. Toxic effects of cadmium (inorganic metal), DDT (organochlorine pesticide), and fluoranthene (polycyclic aromatic hydrocarbon) exposure in sediments (10-day) and seawater (24-h) on juvenile (212-350-microm) hard clams Mercenaria mercenaria were determined. The aqueous 24-h LC(50) values were 0.42 mg/L cadmium (95% CL=0.35-0.45 mg/L), 0.61 mg/L DDT (95% CL=0.40-0.95 mg/L), and 0.65 mg/L fluoranthene (95% CL=0.44-1.23 mg/L). Results of sediment toxicity tests indicated that the 10-day LC(50) values were 1.66 mg/kg cadmium (95% CL=1.21-2.28 mg/kg), 5.8 mg/kg DDT (95% CL=4.8-8.3mg/kg), and 1.75 mg/kg fluoranthene (95% CL=1.38-2.09 mg/kg). Based on comparisons to toxicity data for other marine species, these findings suggest that the juvenile clam is one of the more sensitive species to a variety of contaminants and may be a valuable indicator for potential sediment toxicity.