Antibacterial activity was evaluated on crude extracts of marine bivalves, namely Anadara granosa, Placenta placenta and Pinctada fucata were collected from Vellar estuary, Parangipettai, Tamilnadu, India. The antibacterial activity was carried out against 10 pathogens. The human pathogen S. aureus showed 6 mm. against crude methanol extract of A. granosa and 5mm in Escherichia coli, Vibrio cholorae and Vibrio parahaemolyticus against methanol crude extract of P. placenta. In P. fucata 8 mm. was observed an against Streptococcus pyogens. In shrimp lethality test, the maximum mortalities were observed 80% against 500µg/ml of the crude extract of A. granosa and 0% mortality was observed against 100µg/ml concentration of P. fucata. The results strongly suggest that, the mollusk extracts can be used an antimicrobial agents and further studies.