The distribution and frequency of shell-boring green and blue-green algae in the intertidal at Goa, India were studied. The green alga Gomontia sp. and the blue green algae Hyella caespitosa Bornet et Flahault, H. gigas Lucas et Golubic, Mastigocoleus testarum Lagerheim, Pletonema terebrans Bornet et Flahault, and Phormidium sp. were found in the shells of Placuna placenta, Crassostrea cucullata, Perna viridis and Turritella turritella, Plectonema terebrans occurred most frequently in all shells, and Phormidium sp. occurred least frequently. The presence of chlorophyll b in green algae and its absence in the blue-green algae were used to estimate the contribution of these two groups of algae in shells. Using the ratio of chlorophyll, dry weight and particulate organic carbon (POC) values in cultures of the green alga Gomontia sp. and the blue-green alga Plectonema terebrans, in biomass and POC contribution of these two types of microalgae in shells were calculated.