The amino-terminal sequences and amino acid compositions of the three major and two minor polypeptides constituting the filaments of Spirochaeta aurantia periplasmic flagella were determined. The amino-terminal sequence of the major 37.5-kDa outer layer polypeptide is identical to the sequence downstream of the proposed signal peptide of the protein encoded by the S. aurantia flaA gene. However, the amino acid composition of the 37.5-kDa polypeptide is not in agreement with that inferred from the sequence of flaA. The 34- and 31.5-kDa major filament core polypeptides and the 33- and 32-kDa minor core polypeptides show a striking similarity to each other, and the amino-terminal sequences of these core polypeptides show extensive identity with homologous proteins from members of other genera of spirochetes. An additional 36-kDa minor polypeptide that occurs occasionally in preparations of S. aurantia periplasmic flagella appears to be mixed with the 37.5-kDa outer layer polypeptide or a degradation product of this polypeptide.