Eight strains of Nervous Necrosis Virus (NNV) were used to study on pathogenl
!city and inducing immune response in grouper being cultured in Vietnam. These virus strains having different virulences obtained through assessment of pathogenous levels in susceptible cells and grouper fry/fingerlings, with index TCID50 reached from 10 exponent -2.7 to 10 exponent -3.9, index LD50 reached from 10 exponent -1.5 to 10 exponent -7.5. Of which, four NNV strains namely QN 02, QN 05, QN 07, KH 05 were determined to have high virulence, causing 100 percent groupers died after 3 days post infection. Evaluation of inducing immune response of these virus strains was determined that KH 05 virus strain was highest antigen representative. This virus strain was inactivated by formalin 0.3 percent, NNV KH 05 was emulsified by Montanide ISA 70 that could generate antibody, neutralized with high virulence NNV at day 15th in rabbits and antibody protected 80 - 85 percent in groupers at day 20th after inducing immunity. This result could be used as basis for producing vaccine against NNV disease.