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 the bacteria from swine. In vitro antibacterial activities of the crude extracts prepared from the C. japonica using 80 % methanol were tested for inhibitory activity against the Escherichia (E.) coli (S175), Enterococcus (E.) faecium (ATCC 51558), Salmonella (S.) Typhimurium and Staphylo-coccus (S.) aureus (ATCC 25923) by using broth dilution method. All organisms were incubated in brain heart infusion medium containing 1% extract at 0, 4, 8, 12 and 24 hrs. The 3 days-old piglets were fed an experimental diet supplemented with 1% C. japonica for 1 week. And the change of the coliform bacteria in feces were examined after supplement of C. japonica for 1 week. When the inocula containing $10^2{sim}10^3$CFU/ml of each organism were used the extracts of C. japonica showed various degrees of antibacterial effects on all bacteria tested. The CFU value ($6.3 imes10^8$CFU/ml) of C. japonica for E. coli was decreased 30% compared with vehicle controls ($9.0 imes10^8$CFU/ml) after 8 hrs incubation. The proliferation rate of E. faecium was inhibited about 68% at 4 hrs, 81% at 8 hrs and 76% at 12 hrs after incubation, respectively. The proliferation rate of S. Typhimurium was inhibited about 96% at 4 hrs, 90% at 8 hrs and 72% at 12 hrs after incubation with extracts of C. japonica. The proliferation rate of S. aureus was inhibited 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.