Survival and growth of Penaeus monodon fed the probiont Bacillus S11 in two 90-day culture trials were increased compared with non-treated shrimp. Bacillus S11 also efficiently activated and increased the engulfment of foreign particles (phagocytic activity) as measured by % phagocytosis and phagocytic index (PI) in hemolymph. Phenoloxidase and antibacterial activities increased with age in all shrimp, but were even further increased by probiotic treatment. Following the second trial, after 90 days' culture with and without Bacillus S11 feed additives, shrimp were exposed to pathogenic, luminescent bacteria (Vibrio harveyi). After 10 days' exposure to V. harveyi, probiotic-treated shrimp had significantly greater (p<0.05) survival (54.3%) compared with non-treated shrimp (35.5%). Immune responses were substantial in both treatment groups following the 10-day challenge, but were more pronounced with the probiotic-treated shrimp. The PI was significantly greater with probiotic-treated shrimp (2.7±0.8) compared with controls (0.6±0.3). Bacillus S11 provided disease protection by activating both cellular and humoral immune defenses, as well as presumably providing competitive exclusion in the shrimp's gut.