Growth rate, condition and mortalities in groups of oysters in oyster-beds in the Belon River were studied during 1968 and 1969. The growth rate up to the beginning of the summer was higher in 1969 than in 1968. Variations in the condition of the oysters during the 2 summers were related to the state of development of the reproductive organs and this sexual development was earlier in 1969 than in 1968 because of a sudden rapid rise in temp. Whereas relatively mortalities occurred during 1968, many oyster beds in the River Belon were decimated during 1969. The phenomenon responsible for these mortalities apparently occurred at a particularly critical time; when the oysters were growing rapidly and when most of them were at an advanced stage in the sexual cycle.