Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant Length. Here, we examined hook length and found that it was determined not by molecular rulers but probably by the amount of subunit protein secreted by the flagellar export apparatus. The export apparatus shares common features with the type III virulence-factor secretion machinery and thus may be used more widely in Length determination of structures other than flagella.