Lyophilized powder (BP2FS) of soybeans fermented with Bacillus polyfermenticus KJS-2 (B. polyfermenticus KJS-2) exhibited in vitro antibacterial activities against eight pathogenic bacteria. BP2FS was used as a fodder additive for Oplegnathus fasciatus (0. fasciatus) culture. One group (UFD) of O. fasciatus was fed a commercial fodder, while another group (FD) was fed the same fodder, but including BP2FS ($6{ imes}10^{4}$ cfu $g^{-1}$ fodder), two times daily for 120 days. The mean body weight of the FD group (67.29${pm}$12.62 g) was higher than that of the UFD group (56.56${pm}$8.21 g) after 120 days. The survival rate of FD was 80% compared to 40% for the UFD group. Cumulative mortalities in the FD and UFD groups were 18.95% and 60.98% respectively. B. polyfermenticus KJS-2 was isolated from the intestines of the FD group and the number of viable colonies was estimated to be $1.04{ imes}10^{4}$ cfu $g^{-1}$. Iridovirus and Vibrio vulnificus was detected in the organs of the UFD group but not in the FD group. All of the infected fish showed typical clinical symptoms of hemorrhage in their tail fins. Dissection of the infected internal organs revealed liver congestion and spleen enlargement - typical symptoms caused by iridovirus infection. These results clearly show that BP2FS is highly beneficial in preventing O. fasciatus from iridovirus infection.