The pupose of this study was to determine the effect of betle leaf extract as an antibacterial against koi fish infected with Aeromonas hydrophilla bacteria. The design (CRD) which was divided into 4 treatmenta and 2 repetitions. The treatment in this study was treatment K as a control without adding betel leaf extract, treatment A with a dose of 5 ml of betel leaf extract, treatment B with a dose of 10 ml of betel leaf extract, and treatment C with a dose of 15 ml of betel leaf extract. Based on the research results, it was found that betel leaf extract had a significant effect on the healing of fish infected with Aeromonas hydrophilla bacteria. Because it can be seen from the results of observing the wound in fish given betel leaf extract, the diameter of the wound is narrower. In treatment A with a dose of 5 ml in the fourth week, healing was seen, namely the diameter of the wound began to become narrower, in treatment B with a dose of 10 ml in the third week, healing was seen with narrower wound. In contrast to treatment C which is the best dose, namely with a dose of 15 ml in the second week, a narrower diameter of the wound has begun to be seen or it has begun to fade. For survival in treatment K as much as 50% , treatment A as much as 65%, treatment B as much as 75% and treatment C as much as 90%.

keywords: Effectiveness, Koi fish, Aeromonas hydrophill bacteria