Studying the ability to cause disease of bacteria Aromonas hydrophila in tilapia by injecting bacteria A. Hydrophila into the abdominal cavity. The concentration 3 levels 1x106 cells / ml 1x107 cells / ml, and 1x108 cells / ml for 7 days revealed that can make the dead fish. The experimental group received the abnormality of fish bacterial is atrophy gallbladder, kidney, white spots, and abdominal swelling, which were found. The liver atrophy. Gallbladder, kidney white spots and abdominal swelling of the fish were received bacteria concentration of high, medium and low, and is 100 100 55.55; 100 66.67 33.33;, and, and 33.33 33.33 22.22; 33.33 33.33 11.11, and respectively. The results of this study concluded that A. Hydrophila bacteria have the ability to cause disease as well.