Abstract: In the present study water, ethanol, methanol, acetone, hexane and butanol extracts of two Bivalves, Perna viridis and Crassostrea madrasensis were screened for antibacterial activity. The extracts were obtained from whole body tissue of the animals and tested against 10 different pathogenic bacteria viz., Escherichia coli, Klebsiella oxytoca, K. pneumoniae, Lactobacillus vulgaris, Proteus mirabilis, Pseudomonas aeruginosa, Salmonella typhi, S. paratyphi, Staphylococcus aureus and Vibrio sp. Ethanol extract of P. viridis showed maximum antibacterial activity against E. coli and S. aureus. Ethanol extract of C. madrasensis exhibited highest activity against S. aureus. Water extract of P. viridis and C. madrasensis showed highest activity against E. coli and P. mirabilis, respectively. The 10:10 (methanol: ethanol) fractionated extracts of P. viridis shows highest activity against P. mirabilis (8 mm), 14:6, 4:16 and 2:18 fractions showed prominent activity against P. aeruginosa, E.coli and K. pneumoniae. In C. madrasensis also 10:10 fraction showed highest activity E. coli, P. aeruginosa and S. aureus. The 18:2, 12:8 and 2:18 fractionated extracts of C. madrasensis exhibits effective activity against S. aureus, S. typhi and E. coli. Water, ethanol and methanol extracts showed antibacterial activity against all most all the bacteria tested. Compare to water extracts, ethanol and methanol extracts showed more activity against all pathogens.