A simple, low-cost and efficient bioreactor system for production of anti-infectious hematopoietic necrosis virus (IHNV) substances by bacteria isolated from aquatic environments was devised on a laboratory scale. Whole living cells of anti-IHNV substances producing bacteria, Pseudomonas sp. 51BBW-29 and Alteromonas sp. 48HS-27, were immobilized in calcium alginate beads prepared by 1% sodium alginate, 0.45M CaCl2, and C2Y or MC2Y broth. Eighteen g of the beads were packed into a 25 ml reaction column and incubated at 15℃ for 5 days in a circulating broth medium at a flow rate of 0.35ml/h. After activation of the immobilized bacteria, the beads were incubated in a continuous flow of fresh broth with intermittent exposure to air. In the beads incubated under these conditions, the immobilized bacteria grew well at the gel surface and high viability of approximately 10^10 CFU/bead was retained for at least 6 days from day 8 to 14. During 31 days of the experiment with the bioreactor system, the highest production was 69.3 ⊿IHD50 for Pseudomonas sp. 51BBW-29 (at 14-17 days incubation) and 48.0 ⊿IHD50 for Alteromonas sp. 48HS-27 (at 8-11 days incubation), which were 2.5 and 2.7 times higher than those of conventional batch culture (27.3 and 18.0 ⊿IHD50 for the two strains at 2-5 days incubation). The immobilized bacteria continuously produced high levels of anti-IHNV substances for 15 days (day 5-20)