Crassostrea gigas oysters cultivated in Marennes-Oleron Bay (France), face seasonal variations in the environment: salinity, turbidity, phytoplanktonic production, and the tidal rhythm of emersion. The oysters are maintained at different depths to reduce competition for space (e.g. by settlement of mussel spat). The physiological responses of two groups of oysters to different periods of emersion were studied using a seasonal sampling strategy: one group was on a flat part of the area, and the effect of emersion was studied in a short time survey (less than three hours emersion at low tide); the other was located in a very narrow place with a steep slope. This group was subdivided into three, according to bathymetric position, to study the long term effect of different periods of emersion. Adenylate energy charge (AEC), total nucleotide concentrations and digestive enzyme activities were recorded in each experiment. The effect of short term emersion on energy charge was dependent on season: energy charge stayed high and stable for three hours after emersion in winter (January), but decreased in May and July. AEC did not differ in the long term among groups subjected to daily emersion at different tidal heights, but growth rates of these groups were different. The decrease of AEC after a short emersion would be an indication of the rate of ATP utilization, and is thought to be related to seasonal differences in the metabolism of oysters. This decrease is compensated at each tide, as in a long term survey no differences were observable. So animals from the same location subjected to different amounts of emersion adapt to maintain their energy charge. The effects of emersion and feeding on growth and AEC are discussed with reference to the activities of digestive enzymes in oysters.