Bacterial motility relies chiefly on the rotation of a molecular propeller, the flagellar filament, which is constructed from the protein flagellin. Here, flagellin sequence conservation and diversity is examined in the light of the recently determined flagellar filament structure. As expected, the surface-exposed domains are not conserved. However, the sequences that mediate filament assembly show remarkable conservation, which indicates that all bacterial flagellins are likely to pack into filaments in a similar manner. Flagellins provide a striking illustration of the twin evolutionary themes of conservation and variability.