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¹ Department of Zoology, Faculty of Science, Kasetsart University, 10900, Chatuchak, Bangkok, Thailand DOI : 10.4194/1303-2712-v13_1_22 Viewed : 4184 - Downloaded : 6801 The use of probiotic bacteria for controlling black disease in fairy shrimp, Branchinella thailandensis, was studied. The bacterial antagonistic activities were tested with Aeromonas hydrophila WS1 which is pathogenic to fairy shrimp. The crossstreak method showed that after 48 hours, Bacillus W120 was the strongest inhibitor of A. hydrophila WS1. Bacillus W120 was characterized and confirmed as B. vallismortis by 16S rDNA sequence analysis. Competition, by using nutrient for growth, between B. vallismortis and A. hydrophila WS1 in vitro was studied. The amount of A. hydrophila WS1 co-cultured in nutrient broth with B. vallismortis W120 decreased by 84.14% after 48 hours. The toxicity of B. vallismortis W120 was tested by immersion challenge of fairy shrimp in bacteria suspension at concentrations of 1 ×10⁴, 1 ×10⁵ and 1 ×10⁶ CFU/ml for 72 hours but no mortality was found in any treatment. Fairy shrimp were fed with feed containing B. vallismortis W120 at concentrations of 1 × 10³, 1 × 10⁴ and 1 × 10⁵ CFU/ml for 7 days, and then challenged with A. hydrophila WS1. Fairy shrimp fed with B. vallismortis W120 at every concentration had a significantly higher survival rate than the control group, and the cumulative mortality rates among each B. vallismortis W120-treated group were not significantly different in every experiment period . These results suggest that B. vallismortis W120 can be applied as an effective probiotic in fairy shrimp culture to control the pathogenic bacteria, A. hydrophila WS1, by feeding shrimp this probiotic bacteria daily at a concentration of 1 × 10³ CFU/ml . Keywords : Black disease, fairy shrimp, probiotic, Bacillus vallismortis