The structure and organization of calcium carbonate deposits is studied in species of Halimeda, Udotea, Neomeris (Chlorophyta) and Padina (Phaeophyta). It was found that in Halimeda aragonite deposition takes place outside the cell wall and in the intercellular spaces, while in Udotea aragonitic needles get arranged in layers parallel to the axis of filament within a sheath. In the case of Neomeris, crystallization takes place around the walls of sporangia. In Padina, aragonite crystals are randomly oriented on the surface of cells. A study of the structure and organization of calcium deposits is very much essential in the taxonomy of calcareous algae, which is possible by using SEM. Various studies have proved that this tool offers a great potential. Aragonitic deposition in the members of nemaliales and Padina is studied by Levy and Strauss. McConnell and Colinvaux studied the mineral components of Udoteacean forms