Aeromonas hydrophila is a highly pathogenic bacterium which caused more than 50% nila’s mortality at the pond culture. The purpose of the research was study on the virulence and pathological effect of two A. hydrophila’s enzymes and one toxin haemolycin on a healthy’s nila fish. Lethal Concentration 50 with injection of A. hydrophila on nila was used as preliminary test to determine bacterial density that caused 50% nila’s mortality. The LC50 was 4,9 X 106 cfu/mL determined by Dragsted Bahrens methods. The A. hydrophila virulence test with The Postulat Koch had been effects on healthy’s nila, with clinical sign were reddening, haemorrhagic all over the body surface, and ulcer which caused mortality.