This paper investigates the biological responses of Crassostrea gigas under traditional culture conditions on a mudflat in Marennes-Oleron bay. Summer mortality has been regularly observed in recent years in oysters reared using "on bottom" culture conditions. The present study attempts to provide a better understanding of the mortality phenomenon through biological parameters. Classical 'field parameters' such as mortality and growth rates, and quality index (dry meat weight/ dry shell weight x 1000) were monitored. Additional parameters, as biochemical composition of oyster meat and glucose incorporation capacity, were included as potential new bioindicators. The work highlighted a critical timing (May-June) preceding the summer mortality and characterised by an arrest in lipid synthesis and a decrease in carbohydrate content. During this period, growth (especially gonad maturation) either slowed down significantly or even stopped. The first mortality event occurred during a growth renewal period at the end of June. Short-term analysis (15 days) provided information to identify such responses which may indicate a physiological stress and demonstrating the need for further investigation. The seasonal food availability (estimated from chlorophyll a levels) did not facilitate the mortality understanding which occurred after water temperature went above 18-19 T. Nevertheless, this study shows carbohydrate anabolism contributed in the physiological stress leading to mortality events.