Methanol: water (1:1), methanol: dichloromethane (1:1) and acetone extracts of molluscs comprising 77 whole body, four inks, four opercula, 10 egg masses, and 10 digestive glands were screened for antimicrobial activity on marine biofilm bacteria. The methanol: water (1:1) whole body extracts of Nerita albicilla and Nerita oryzarum showed broad spectral inhibitory activity against 93% and 95% of the 40 biofilm bacteria. The egg masses from 10 gastropods showed activity against more than a quarter of the biofilm bacteria. The methanolic extract of Chicoreus virgineus, Chicoreus ramosus egg masses, and acetone extract of the egg mass of Rapana rapiformis showed broad-spectrum antibacterial activity against all the 40 biofilm bacterial strains. The activity in gastropod egg masses was localized to their internal matrix. Overall screening showed activity in 38.1% of the methanolic extracts followed by 13.3% of methanol: water, 12.4% of methanol: dichloromethane, and 3.8% of acetone extracts. Gastropods showed good activity when compared with bivalves and cephalopods.