Total bacterial count from gastrointestinal tract (GI tract) of black tiger shrimp and their culture water were 1.2x10 6-3.4x10 7 cfu/gm and 1.0x10 4-4.4x10 5 cfu/ml, respectively. Bacterial flora of their GI tract found were Bacillus sp., Vibrio sp., Aeromonas sp., Pseudomonas sp., Klebsiella sp. and Staphylococcus sp. Among them only Bacillus S11 has the ability of inhibiting some strain's growth of pathogenic bacteria to human ; such as, Bacillus cereus, Bacillus subtilis, Bacillus megaterium, Listeria monocytogenes, Staphylococcus aureus and Escherichia coli and pathogenic bacteria to shrimp; such as, Pseudomonas aeruginosa, Aeromonas hydrophila, Vibrio alginolyticus, Vibrio anguillarum, Vibrio harveyi, Vibrio parahaemolyticus and Vibrio cholera. Bacillus S11 clearly produced antimicrobial substance starting from the late log to stationary phases of their growth. After the examination of morphology and biochemical test, Bacillus S11 was tentatively identified as Bacillus mycoides. Being probiotics, Bacillus S11 was mixed in shrimp feed for feeding black tiger shrimp in closed recirculating water system and it was found that growth and survival rate of shrimp fed with probiotics were higher than those without probiotics feeding significantly at the level of p<0.05. The growth rate of shrimps in control group and treated group of Bacillus S11 in the pattern of Fresh cells, Fresh cells in NSS and Lyophilized cells within 21 days were 0.73, 1.29, 1.17 and 1.16 grams, respectively. Whereas, their survival rate at the end of the experiment were 15.8, 38.3, 31.6 and 30.0%, consecutively. After challenging the control and the treated group with Vibrio harveyi 10 days their survival rate were 26% and 100%, respectively, with the significant difference of p<0.05 by Duncan's multiple range test
