Abstract: To study the infection mode of Vibrio parahaemolyticus on zebrafish and the immune effects of soybean isoflavones on the Vibrio, two groups of experiments were performed. Ten zebrafish in each group were challenged by intramuscular injecting, surgical injuring and water soaking with different concentration of the Vibrios, respectively. The results show that intramuscular injection could cause death in high concentration of the bacteria. The LC₅₀ is 5.63×10⁷ CFU · mL^-1 (48 h).No death was observed in immersion treatment while low mortality occurred in wounding treatments. As a result, intramuscular injection is an important infection mode of Vibrio parahaemolyticus on zebrafish. Moreover, the mortality is much lower at low water temperature (16℃) than that at high water temperature (28℃) in muscular injection treatment, showing that water temperature and the bacterial concentration are important factors for mortality in the injection group. For the immune effect test of soybean isoflavones, the animals were administered feeds with additive of soybean isoflavones at concentration of 0% (as a control), 0.5%, 1% and 2%, respectively for fourteen days. The fishes were subsequently subject to muscular injection of Vibrio at the concentration of 3.6×10⁹ CFU · mL^-1. The results indicate that 1% soybean isoflavones can resist infection, suggesting that it is good to reduce diseases and to be a new immunostimulants in aquaculture practice.