AimsThe aim of this study was to investigate functions of flagellar genes fliC2, fliC12, fliA and flhDC in a bacterial fish pathogen Edwardsiella tarda. Methods and ResultsIn this study, functions of flagellar genes, fliC2, fliC12 (fliC1+fliC2), fliA and flhDC (flhD+flhC) of Edw.tarda H1 were analysed by constructing in-frame deletion mutants respectively and complementary strains fliC2(+) and fliA(+). Electron microscopy revealed that in-frame deletion of fliC12, fliA and flhDC significantly impaired the number and length of flagellar filaments, resulting in loss of both swimming and swarming motilities of the bacteria. In addition, compared to the wild-type strain and complementary strains, the flagellum-impaired mutants exhibited reduced biofilm formation ability, showed decreased ability in adherence and internalization to Epithelioma papulosum cyprini (EPC) cells and reduced pathogenicity to zebrafish. ConclusionsThese results indicated that fliC12, fliA and flhDC of Edw.tarda played essential roles in flagellar filaments structure, bacteria motility, biofilm formation, adherence, internalization and pathogenicity of this bacterium. Significance and Impact of the StudyThis study revealed that flagella function in facilitating virulence and it may provide a new target for vaccines against Edw.tarda infection.