ABSTRACT Bacillus spp. were isolated from the intestine of black tiger shrimp Penaeus monodon which were collected from the Gulf of Thailand at Chonburi province area during October 2005 to September 2006. The bacterial antagonist activity were tested with aquacultural pathogenic bacteria. The cross streak method results showed that Bacillus W803 and Bacillus W120 could inhibit Aeromonas hydrophila AQAH after 24 hours.