The clam Meretrix meretrix, the green mussel Perna viridis and the oyster Crassostrea madrasensis exist in the Ennore Estuary. The ' bar' is open throughout the year and as a result except during the Northeast Monsoon period when there is significant fall in salinity, higher salinity as in the inshore Waters are prevalent. The present investigations were designed and aimed at studying the salinity tolerance of the above mentioned bivalves. In the experiments conducted they were subjected to a series of salinity changes ranging from 0 to 35 %„. The rates of mortality of the animals in different salinity media were noted at the end of 24 hours for a consecutive period of ten days. It was found that the bivalves were tolerant to the salinity change of the experimental media when the survival rate was 50% and above at the close of this period. The results obtained show that Crassostrea madrasensis, Meretrix meretrix and Perm viridis can tolerate an approximate low salinity of 7, 13 and 16%, respectively under laboratory conditions.