EmpA is an extracellular metalloprotease secreted by Vibrio anguillarum.For better understanding its role in the patho-genicity of V.anguillarum strain M3,empA insertion mutant was constructed.In the mutant it decreased in extracellular proteolytic activity,swarming motility,hemolytic activity and virulence on turbot(Scophthalmus maximus).Significant decline(by 5-fold)of extracellular proteolytic activity and similar growth curve between mutant and wild type strains indicated that EmpA was the major extracellular protease of M3.LD50 of mutant increased by 38-fold compared with wild type.No pro-EmpA was detected in the su-pernatant of culture,indicating that EmpA autolyzed to mature protein after 24 h.Secretion of EmpA in M3 was similar to that in NB10 strain.Attenuated virulence of mutant was similar to that of M93Sm strain.It was demonstrated that specific operation of EmpA was different from that in previous studies and EmpA contributed to the swarming motility and hemolytic activity in V.an-guillarum strain M3.The results provides insight into understanding the function of EmpA and its potential application in vaccine development. EmpA is an extracellular metalloprotease secreted by Vibrio anguillarum. For better understanding its role in the patho-genicity of V. anguillarum strain M3, empA insertion mutant construct in. The mutant it decreased in extracellular proteolytic activity, swarming motility, hemolytic activity and virulence on turbot (Scophthalmus maximus) .Significant decline (by 5-fold) of extracellular proteolytic activity and similar growth curves between mutant and wild type strains indicates that the major is extracellular protease of M3.LD50 of mutant increased by 38-fold compared with wild type. No pro-EmpA was detected in the su-pernatant of culture, indicating that the EmpA autolyzed to mature protein after 24 h. Secretion of EmpA in M3 was similar to that in NB10 strain. Attenuated virulence of mutant was similar to that of M93Sm strain. It was said that specific operations of EmpA was different from that in previous studies and EmpA contributed to the swarming motility and hemolytic activity in V. an-guillarum strain M3.The results provides insight into understanding the function of EmpA and its potential application in vaccine development.