Clostridium tetani ATCC 19406 was investigated with regard to the flagellar filaments produced by this anaerobic species. Flagellar filaments were removed from the cell bodies by hydrodynamic shear forces and purified by differential centrifugation. Exposure to acid was shown to result in disaggregation of the flagellar filaments into a preparation of flagellar protein containing 3.5% carbohydrate. The protein was judged homogeneous after examination by acrylamide gel electrophoresis in the presence of 4 M urea at several pH levels, and was shown to have a molecular weight of 35,000 daltons. Amino acid analyses indicated the absence of cysteine and tryptophan and a preponderance of acidic residues, epsilon-N-methyllysine was shown to be absent and the N-terminal amino acid was identified as alanine. Analysis of the C-terminal region indicated the sequence -Leu-Leu-Arg. These findings indicated that the obligate anaerobe C. tetani produced flagella filaments similar to previously studied filaments of aerobic and facultatively anaerobic bacteria.