Oyster culture is of great economic value for France. A large part of oyster production is situated in the Southwest of France, in the Bay of Marennes-Oléron. The last 20 years a phenomenon called summer mortality of the oyster species Crassostrea gigas occurs in the bay. A high percentage of the oysters die during the spring and summer when maturation takes place. Pollution, predation and iIlness seem not the primai cause of summer mortality in the Bay of Marennes-Oléron. Neither can overstocking problems explain the occurring mortalities. Two different culture types are used in the bay. The first is seeding the oysters on the sediment (on bottom culture), the second is placing the oysters in bags on tables made of iron bars (on table culture). 1 compared the mortality rate for the different culture types in 1998 and 1999 for 4 sites at the banks of Ronce-Perquis. Furthermore, 1 studied temperature, pH, dissolved oxygen and salinity in the field. Tests in the laboratory were developed to test the influence of different abiotic conditions (temperature, pH and dissolved oxygen) on mortality rate in oysters. Biochemiql analyses were done on oysters at the start of the experiment and on the oysters that died during the experiment. In 1998 1 did not find a significant difference in accumulated mortality rate, averaged over four sites, between the two culture types. In 1999 accumulated mortality rate, averaged over four sites, is significantly different for the two culture types. The significant difference per period averaged over sites occurs in the periods from half May till half July. This is also the period in which maturation of the oyster takes place. I did not find a trend in temperature difference between the two culture types in 1998 and 1999. Dissolved oxygen and salinity data from site 3 in 1999 did not show a difference in trend between the height of the two culture types during April tilI June. However pH recordings from site 3 in 1999, showed that pH was always higher at table culture height than at ground culture height in April till June. The laboratory results showed that placing mature oysters (that are rich in carbohydrates) every day in a dry oyen at 35° C for three hours followed by three hours in a tank with seawater of pH of 9.5-10 (value also found in the field) causes a significant mortality rate.  The group of oysters that had only the temperature 'stress' did not show a significant mortality rate. Neither did the group of oysters that was subjected to the temperature 'stress' followed by one hour in a tank  with seawater with 4-5 mg/lof dissolved oxygen (value also found in the field) show a significant effect on mortality rate. 1 did not find a significant difference in mortality rate of the same groups during a period of starvation after the 'stress' experiments. Oysters from the bay that have a lower carbohydrates percentage might however show significant mortality rate. The biochemistry of the oysters that died in tbis stress  experiment differ significantly in lipids and glycogen but not in dry weight and carbohydrates from the oysters at to. Both lipid concentration and glycogen concentration is lower in the oysters that died showing an influence of stress on the energy resource of glycogen that is also used when the oyster closes its shell to withstand stress. Overall pH stress can he of great importance for mortality rate difference between the two types of Crassotrea gigas culture in the Bay of Marennes-Oléron. A combination of different abiotic stress can effect glycogen content (used for anaerobic respiration during low tide) in mature oysters and therefore play an important part in summer mortality.