Before 1960 the normal level of mortality of oysters maintained in beds in the Belon river during the summer was in the region of 15%. During several years in the period 1960-1970 much higher mortalities, up to 70% of the stock, were experienced. The circumstances of these mortalities and the appearance of the oysters themselves indicated that no parasitic disease or physiological deficiency was involved. 2 possible caused for the mortalities have been suggested, pollution or a sudden change in the physico-chemical environment. Although some industrial pollution of the River Belon occurs, This is very slight and the oyster beds most affected have been the ones furthest from the source of pollution. Similarly, the River Aven, which has a common mouth with the Belon, is more heavily polluted, but there is no evidence that Aven water gets into the Belon in any significant amount. On the other hand several of the heavy mortalities have followed periods of unusually stormy weather accompanied by fogs. Also, in the period 1968-1970, the penetration of the Belon in early July by a mass of cold water with characteristics similar to offshore water has been followed by oyster mortalities.