Flagellin proteins derived from Pseudomonas syringae pv. tabaci 6605 and flg22Pa (QRLSTGSRINSAKDDAAGLQIA), one of the microbe-associated molecular patterns (MAMP) in bacterial flagellin, induce cell death and growth inhibition in Arabidopsis thaliana. To examine the importance of aspartic acid (D) at position 43 from the N-terminus of a flagellin in its elicitor activity, D43 was replaced with valine (V) and alanine (A) in P. syringae pv. tabaci flagellin and flg22Pta. The abilities of flagellins from P. syringae pv. tabaci D43V and D43A to induce cell death and growth inhibition were reduced, whereas the abilities of flg22PtaD43V and flg22PtaD43A were abolished. These results indicate that D43 is important for elicitor activity in P. syringae pv. tabaci. When tobacco plants were inoculated with each bacterium by the spray method, both P. syringae pv. tabaci D43V and D43A mutants had remarkably reduced ability to cause disease symptoms. Both mutants had reduced or no swimming and swarming motilities and adhesion ability. In P. syringae pv. tabaci D43V, little flagellin protein was detected and few flagella were observed by electron microscopy. These results indicate that mutant flagella are unstable and that flagellar motility is impaired. Thus, the amino acid residue required for MAMP activity is important for the intrinsic flagellar function.