This study was designed to assess in situ the effects of sediment nearness on Crassostrea gigas metabolism. One year-old oysters were reared for 5 months (April-August) in plastic bags on metallic frames at 10 and 60 cm off the sediment at the beginning of April. The management of the energetic resources (storage, consumption) was estimated with respect to biochemical changes of proteins, lipids and carbohydrates. In order to evaluate the effect of the sediment nearness at cellular level, the metabolic rate was evaluated by assessing pyruvate kinase (PK) and phosphoenolpyruvate carboxykinase (PEPCK) enzyme activities and electron transport system (ETS) activity. The metabolic pathways were assessed by measuring ATP and intermediary metabolites such as alanine, succinate and aspartate. Despite similar survival and growth at the two rearing levels, the protein, lipid and carbohydrate content were significantly lower in the oysters close to the bottom. In the oysters reared at 10 cm, PK activity was also significantly reduced and a significant negative correlation between alanine content and PK activity was revealed. The sediment nearness decreased significantly the ATP content in the oyster. There was a significant relationship between ATP and aspartate in these oysters. The use of aspartate contributed to complete the metabolic pathways along with carbohydrate allowing to maintain the same biological performance as the oyster reared far from the bottom. The energy state (proteins, lipids, carbohydrates and ATP contents) could suggest that the oysters close to the sediment were fed less but the metabolic enzyme activities allow to suggest an oxygen deficiency. However, the reduced PK activity, the absence of PEPCK activity stimulation and the utilization of aspartate suggest a transition stage to anaerobiosis.