Bacterial motility is most likely a critical factor for rhizobium to chemotactically colonize on the root surface prior to infecting leguminous plant hosts. Several studies of the rhizobium flagellar filament have been reported, but little is known about the rhizobium flagellum hook. To investigate the roles of the hook protein in flagellum synthesis in Mesorhizobium tianshanense, the hook protein-encoding gene flgE of M. tianshanense was amplified by PCR and sequenced. Comparison of the deduced amino acid sequences revealed pronounced similarities in Domain 1 and lower similarities in Domain 2, which are supposed to be related to hook structure assembly and antigenic diversity, respectively. The level of transcription of flgE increased along with the cell growth and reached its maximum at the middle log phase. Disruption of the flgE gene caused a flagellar-less phenotype, thereby causing complete loss of swimming ability, modified nutrient-related swarming ability and biofilm formation. Moreover, the absence of flagellar caused decreased bacterial attachment on the root hair, suggesting that flagellar is involved in the early stage of symbiosis process.