Xanthomonas campestris pv. campestris (Xcc) is a gram-negative bacterium. It can infect cruciferous plants and cause black rot disease. Xcc secrets various type of enzymes and polysaccharides which are essential for the bacterial virulence. Previous data from our lab have indicated that Xcc has a single polar flagellum. Biogenesis of the flagellum is regulated by the three-tier regulation of sigma factors RpoD, RpoN and FilA. A basal body composed of a flagellar type-III secretion system is essential for the secretion of the flagellin protein FliC. In other Gram-negative bacteria, FliF protein is the first element constituting the type III secretion system. Homolog of fliF was identified in the complete sequenced genome of Xcc. The aim of this study is to characterize the biological function of fliF gene in Xcc. Mutant of fliF was constructed by insertional mutagenesis. Phenotypes of the fliF mutant were analyzed by motility assay and pathogenic analysis. Regulatory function of fliF was analyzed by western blot. Interactions between FliF and other flagellar proteins were studied with the yeast-two hybrid system. The results demonstrated that 1) FliF is essential for the motility; 2) FliF is not involved in the pathogenicity; 3) FliF positively regulates the production of FliC, and 4) FliF interacts with FlhF protein.