Haryanti'r, Ketut sugama"r, seiichi rsumura"'), and roshitaka Nishijima""r ABSTRACT . Alteromonas sp. (coded as BY-9) bacterium isolated from the coastal water of Gondol Bali, fndonesia was purified and mass produced, then inoculated into the rearing media of penaeus monodon farvae as a probiotic agent. Nauplii of Penaeus monodon were reared in concrete tanks capacity of 18 m3 each with a density of 100 nauplii/L. The larvae were reared with and without Alteromonas sp. up to stage 10 postlarvae. At day 18 the growth and survival rates of the larvae reared with Alteromonas sp. were significantly higher than in the control (p< 0.0S). probiotic treatment produced an those average survival rate of 44.90 * 8.30 %, whereas the control produced 10.15 t1 .91o/o survival rate. Application of Alteromonas sp. bacterium strain at a densiiy of 106 cfu/ml to the culture media of larvae resulted in befter growin of larvae and exhibited antagonistic activity against vrbrlo, especially the luminous bacterium (vibno harueyi).