is a Gram-negative bacterium with a monotrichous flagellum that causes the human disease cholera. Flagellum-mediated motility is an integral part of the bacterial life cycle inside the host and in the aquatic environment. The flagellar filament is composed of five flagellin subunits (FlaA, FlaB, FlaC, FlaD, and FlaE); however, only FlaA is necessary and sufficient for filament synthesis. is transcribed from a class III flagellar promoter, whereas the other four flagellins are transcribed from class IV promoters. However, expressing from a class IV promoter still facilitated motility in a strain that was otherwise lacking all five flagellins (Δ). Furthermore, FlaA from (FlaA; 77% identity) supported motility of the Δ strain, whereas FlaA from (FlaA; 75% identity) did not, indicating that FlaA amino acid sequence is responsible for its critical role in flagellar synthesis. Chimeric proteins composed of different domains of FlaA and FlaD or FlaA revealed that the N-terminal D domain (D) contains an important region required for FlaA function. Further analyses of chimeric FlaA-FlaD proteins identified a lysine residue present at position 145 of the other flagellins but absent from FlaA that can prevent monofilament formation. Moreover, the D region of amino acids 87 to 153 of FlaA inserted into FlaA allows monofilament formation but not motility, apparently due to the lack of filament curvature. These results identify residues within the D domain that allow FlaA to fold into a functional filament structure and suggest that FlaA assists correct folding of the other flagellins. causes the severe diarrheal disease cholera. Its ability to swim is mediated by rotation of a polar flagellum, and this motility is integral to its ability to cause disease and persist in the environment. The current studies illuminate how one specific flagellin (FlaA) within a multiflagellin structure mediates formation of the flagellar filament, thus allowing to swim. This knowledge can lead to safer vaccines and potential therapeutics to inhibit cholera.