The purpose of this study was to examine the responses of the oyster C. gigas to oxygen levels at subcellular and whole-organism levels. Two experiments were carried out. The first experiment was designed to measure at 15°C, 20°C and 25°C for 20 hours the clearance (CR) and oxygen consumption (OC) rates of oysters exposed at different concentrations of oxygen. The goal of this first part was to estimate the hypoxic threshold for oysters below which their metabolism shifts toward anaerobiosis, by estimating the oxygen critical point (PcO2) at 15, 20 and 25°C. The second experiment was carried out to evaluate the metabolic adaptations to hypoxia for 20 days at three temperatures: 12°C, 15°C and 20°C. The metabolic pathways were characterized by the measurement of the enzymes pyruvate kinase (PK) and phosphoenolpyruvate carboxykinase (PEPCK) and the alanine and succinate content and the adenylate energy charge (AEC). The respiratory chain functioning was estimated by the measurement of the activity of the electron transport system (ETS). The values of PcO2 were 3.020.15, 3.430.20 and 3.280.24 mg O2 L-1 at 15, 20 and 25°C, respectively. In whole oysters, hypoxia involved the inhibition of PK whatever the temperature but PEPCK was not stimulated. Succinate accumulated significantly only at 12°C and alanine at 12 and 15°C. A negative relationship between the PK activity and the alanine content was found only in hypoxic oysters. Lastly, hypoxia increased significantly the activity of ETS. With high PcO2 values, the metabolic depression occurred quickly showing that oysters had a low capacity to regulate their respiration when oxygen availability is reduced, particularly in summer.