A bacterium was isolated from the mid-gut of healthy black tiger shrimp, Penaeus monodon, based on a large zone of inhibition in mixed culture on solid medium. The isolate was a Gram-positive, motile spore former, with an optimum pH range for growth in tryptone soya broth containing 2% NaCl of between pH 6 and 9. The bacterium was highly salt tolerant with concentrations between 0% and 8% having no detrimental effect on growth. The isolate was identified as Bacillus pumilus based on physiological capabilities using the API50CHB and Biolog systems. Amplification and sequencing of the 16S rRNA gene followed by phylogenetic analysis confirmed its identity. The Bacillus pumilus isolate was strongly inhibitory against the marine bacterial pathogens Vibrio alginolyticus, V. mimicus and V. harveyi, and weakly inhibitory against V. parahaemolyticus in cross-streaking assays on solid medium. The organism was marginally self-inhibitory, and inhibited B. licheniformis and B. subtilis. The suitability of the B. pumilus isolate for use as a probiotic in farmed shrimp was further supported by the absence of any of the known B. cereus enterotoxin genes. Based on these in vitro results, in vivo safety and efficacy trials are underway to determine suitability of the novel strain as a commercial probiotic.