Effect of probiotic Bacillus strain S11 on the growth and immunoenhancement either by cellular or humoral defenses in Penaeus monodon were shown after feeding black tiger shrimp in closed-recirculating pond for 90 days. Shrimp weight and their survival in postlarvae shrimp but not in juvenile ones were significantly higher (p<0.05) than those of control group. Bacillus strain S11 could also efficiently activate and increase the engulfment of foreign particles (phagocytosis) as shown by the significant increase of %phagocytosis, phagocytic index and phenoloxidase at the level of p<0.05. When performing for the challenge test by Vibrio harveyi strain 1526 for 10 days, accumulation of mortality (45.7%) was lower in shrimp fed probiotic supplement as compared to a control one (64.5%). Furthermore %phagocytosis, phagocytic index and average number of the beads ingested per cell of probiotic treated shrimp were significantly higher (p<0.05). The effect from the challenge test by Vibrio harveyi strain 1526 on immune activition in both probiotic and control groups was observed and found that their total hemocytes decreased but %phagocytosis, phagocytic index and antibacterial activity increased significantly (p<0.05)