Portuguese oysters, C. angulata from brood ponds at Arcachon (salinity 27-28ppt) were exposed to lowered salinities and their valve activity continuously recorded. Down to 2 ppt, normal activity was resumed within 24 hours, between 8 and 11%, oysters, survived and adapted slowly, below 8%, oysters died. Sexually mature oysters are more susceptible and may die at 10 to 8% salinity, perhaps because the gonad is full of gametes. Oysters from different localities probably respond differently to changed salinity.