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Interest in marine organisms as potential sources of bioactive agents has increased in recent years. The red seaweed, Callophyllis (C.) japonica, is abundant in the coastal regions of Jeju Island in South Korea. A previous study shows that C. japonica extracts have antioxidant activity and radioprotective effects. In this study, an methanol extract of C. japonica was tested whether it has antibacterial effects against porcine bacteria. the in vitro antibacterial activities of 80% methanol crude extracts prepared from the C. japonica were tested for inhibitory activity against the Escherichia (E.) coli (S175), Enterococcus (E.) faecium (ATCC 51558), Salmonella (S.) Typhimurium and Staphylococcus (S.) aureus (ATCC 25923) using broth dilution method. All organisms were incubated in brain heart infusion (BHI medium) containing 1% extract at 0, 4, 8, 12 and 24 hrs. Suckling piglets were fed an experimental diet supplement with 1% C. japonica for 1 week. And the change of fecal coliform bacteria were examined after supplement of C. japonica for 1 week. The inocula containing 102-103CFU/ml of each organism were used. The extracts of C. japonica were shown to be antibacterial effects on all bacteria tested. The CFU value (6.3×108CFU/ml) of C. japonica for E. coli was decreased 30% compared with vehicle controls (9.0×108CFU/ml) after 8 hrs incubation. For E. faecium was inhibited proliferation rate about 68% at 4h, 81% at 8 hrs and 76% at 12 hrs after incubations, respectively. For S. Typhimurium was inhibited proliferation rate about 96% at 4h, 90% at 8 hrs and 72% at 12h after incubation with extracts of C. japonica. For S. aureus was inhibited proliferation rate more than 90% each time courses. Conclusively, a red seaweed extract of C. japonica was found to be effective against a number of gram negative and gram positive bacteria such as E. coli, E. faecium, S. Typhimurium, and S. aureus. The number of coliform bacteria was increased in the 1% C. japonica treated group, as compared to those of controls. This result suggests that C. japonica extracts be added as an effective natural antibacterial agent. The precise mechanism of antibacterial effects and its application on swine industry remains to be further studied.