Salinity tolerance and rate of filtration of the pearl oyster Pinctada fucata were studied during 1975-77. The oysters were experimented in salinities ranging from 14 to 58 %. Although a truly marine form, the pearl oyster has been found to tolerate a wide range of salinity from 24 to 50%, for short durations of 2-3 days. Salinities below and beyond the a ove range caused mortality of oysters. The rate of mortality in the dilutions of 16,15 and 14 %o were 10, 50 and 100% respectively and mortality in higher concentrations of 52, 55 and 58 %o were 67,100 and 100% respectively. In the normal sea water (salinity 34%,) the removal of neutral red in solution was 52.1 % in 2 hours and 92.6% in 8 hours. The rate of filtration was low in dilutions and the total filtration was bel w 25 % in the salinities of 14 and 20 %,. In the higher concentrations, filtration was 49.5, 53.7 and 41.8 % in the salinities of 44,50 and 57 %, respectively at the end of 4 hours. In the Gulf of Mannar, where a pearl culture farm is located, the normal salinity range during 1974-76 was 32.15-35.58 %.. An unusual incidence of dilution of sea water down to 15.69/^, occurred in the farm at Veppalodai in November 1977 due to heavy rainfall and floods m the rivers caused by an active north-east monsoon. However, this did not affect the oysters as the low saline condition did not last for more than a day.