Sediment samples from fresh water, estuarine and tidal creek environments of Krishna Delta and core samples off Krishna River Confluence collected during the 18^th cruise of "INS Kistna" are analysed for the clay minerals by X-ray diffraction technique. The predominant clay mineral is montmorillonite. Minor amount of Kaolinite is present in Krishna River channel and deltaic sediments. Traces of illite are found in marine sediments off Krishna River in addition to the above clay minerals. The montmorillonite is subjected to a minor chemical change in the estuarine environment from Ca'+ montmorillonite to Na+ montmorillonite with no marked structural alteration. Such a transformation of montmorillonite towards progressively saline water is attributed to the increasing concentration of Na+ions in the overlying waters. The chemical composition of the source rocks and the climatic conditions seem to be the important factors controlling the type of the clay minerals in the Krishna deltaic and marine sediments. The above observations indicate that diagenesis is not an effective process in determining the type of clay minerals in these sediments.