Combined effects of temperature and a toxic metal, cadmium, on energy metabolism were studied in a model marine bivalve, the eastern oyster Crassostrea virginica, acclimated at 20, 24 and 28°C and exposed to 50 µg/L of cadmium. Based on the concept of oxygen-limited thermal tolerance in aquatic ectotherms, we show that a persistent pollutant, cadmium, can have detrimental effects on oyster populations by reducing thermal tolerance due to impaired energy metabolism and elevated costs of basal maintenance. At 20 and 24 C but not at 28°C standard metabolic rate of Cd-exposed oysters increased compared to respective controls. Furthermore, highest respiratory time activity was observed in Cd-exposed oysters at 28°C. The results point at a mismatch between oxygen demand and delivery supported by preliminary results that show a decrease in hemolymph oxygen partial pressure during an acute temperature rise (4°C/10h) in Cd-exposed but not control animals (20°C acclimated animals).At 28°C but not at lower acclimation temperatures glycogen content in mantle tissue of Cd-exposed oysters decreased significantly (12.2+4.2mg/g to 5.4+1.3mg/g over 14 days cadmium exposure) suggesting that the combined stressors may increase dependence on glycogen to support maintenance metabolism. Glycogen supplies energy for reproduction as well as for anaerobic metabolism. In line with the continuously decrease in condition and the nearly 50% mortality observed after 14 days of cadmium exposure in oysters acclimated to 28°C we conclude that coupled with the additional stress of cadmium, oysters are less tolerant to cope with increasing energy expenditure with increasing temperature resulting in onset of critical temperature thresholds at lower temperatures.This work was supported by the National Science Foundation IBN-0347238 and Alexander von Humboldt Foundation.