Bivalve molluscs Teredo teredo and Macoma birmanica secrete CaCO 3 in the form of calcite and aragonite respectively, in the mud flats of Hugli estuary and their magnesium contents in the skeletal carbonate were 4.07% (0.168 mole%) for calcite and 0.17% (0.007 mole%) for aragonite. The stoichiometric solubility products (pK * s p ) of biogenous calcite and aragonite in estuarine waters of the Hugli river were determined as function of temperature (5°-40°C) and salinity (0.109-31.73 10 - 3 ) at atmospheric pressure.