ABSTRACT Spirochetes have periplasmic flagella composed of a core surrounded by a sheath. The pathogen Leptospira interrogans has four flaB (proposed core subunit) and two flaA (proposed sheath subunit) genes. The flaA genes are organized in a locus with flaA2 immediately upstream of flaA1 . In this study, flaA1 and flaA2 mutants were constructed by transposon mutagenesis. Both mutants still produced periplasmic flagella. The flaA1 mutant did not produce FlaA1 but continued to produce FlaA2 and retained normal morphology and virulence in a hamster model of infection but had reduced motility. The flaA2 mutant did not produce either the FlaA1 or the FlaA2 protein. Cells of the flaA2 mutant lacked the distinctive hook-shaped ends associated with L. interrogans and lacked translational motility in liquid and semisolid media. These observations were confirmed with a second, independent flaA2 mutant. The flaA2 mutant failed to cause disease in animal models of acute infection. Despite lacking FlaA proteins, the flagella of the flaA2 mutant were of the same thickness as wild-type flagella, as measured by electron microscopy, and exhibited a normal flagellum sheath, indicating that FlaA proteins are not essential for the synthesis of the flagellum sheath, as observed for other spirochetes. This study shows that FlaA subunits contribute to leptospiral translational motility, cellular shape, and virulence.