Filaments of the flagellum of the halophilic archaeon Halobacterium salinarum consist of five flagellins: A1, A2, B1, B2, and B3, which are encoded by five genes localized in tandem in twoflgA and flgB operons. While the role of flagellins A1 and A2 has been determined, the role of the proteins, B operon products, is still unclear. A mutant strain of H. salinarum with deleted A and B flagellin genes (deltaflgAdeltaflgB) has been obtained for the first time. This strain has been used to create and analyze the strains carrying only individual B1 or B3 flagellin genes. Cells of the deltaflgAdeltaflgB strain were shown to have short filamentous formations, 7-8 nm thick, which we have named as X-filaments. It has been shown that X-filaments consist of a protein immunologically related to flagellins A and B. Expression of the B1 and B3 genes is suppressed in the absence of A1, A2, and B2. It has been shown that flagellins B1 and B3 cannot be substituted for flagellin B2 upon the formation of a curved hook-like structure, which serves as a connecting element between the flagellar filament and the motor axis. The multicomponent nature of flagella is discussed in the light of their possible involvement in other cell processes besides providing motility.