Vibrio mimicus is the causative agent of ascites disease in aquatic animals.It was found that there was an adhesion motif at amino-terminal of OmpU protein,and OmpU-specific antibody exhibited significant inhibition effects on the adhesion of V.mimicus to cells in our previous studies,but adhesion function of OmpU protein is still unclear.To investigate the role of OmpU protein from V.mimicus in cell adhesion and virulence,the OmpU gene deletion mutant of V.mimicus 04-14 strain was constructed by the homologous recombination.A recovery mutant was obtained by introducing a low-copy plasmid strain.PCR and sequence analysis confirmed that OmpU gene was successfully deleted in the mutant and restored in the complementary strain.We compared the differences of genetic stability,growth and biochemical characteristics,adhesion ability and virulence between the gene deletion mutant and wild strains,as well as the complemented strain.The results showed that the mutant strain stabilized genetically and grew slower than the wild strain.There was no significant difference between them in morphology,culture and biochemical characteristics.Compared with wild strain,deletion of OmpU gene significantly reduced bacterial adhesion to EPC cells monolayers by 66.6%,but the adhesion ability was restored in the complementary strain.Furthemore,the 50% lethal doses(LD₅₀)of the mutant strain was four times lower than those of the wild strain and complementary strain.These data indicate that the OmpU protein is involved in both adhesion and virulence in V.mimicus.The adhesion function of OmpU protein from V.mimicus and its role in virulence were comfirmed for the first time.