1. 1. A shell thickening study was carried put on Anodonta cygnea molluscs kept in acidic and alkaline water. 2. 2. In acidic water, a Calcified pellicle was formed on the inner face of the shell due to intense secretion of the organic matrix and deposition of calcium mobilized from the mantle reserves. 3. 3. A slightly lower pH of the haemplymph, reduced number of calcium microspherules in the mantle and higher soluble calcium concentration was observed as compared to controls. 4. 4. An intense cell proliferation in the outer mantle epithelium and a great amount of glycogen in the cells was also observed. 5. 5. In alkaline water no calcified pellicle was observed on the shell but a great increase in the number of microspherules was found. The soluble calcium concentrations and pH of the haemolymph were not significantly different from controls.