Flagella of the haloalkalophilic archaebacteriumNatronobacterium magadiiwere purified and characterized. The diameter of the flagella was 10 nm. It was shown that the flagella consist of four major proteins with molecular weights of 105 000, 60 000, 59 000, and 45 000. With decreasing NaCl concentration, the flagella dissociated into protofilaments. The structure of dissociated flagella and individual flagellins was studied by limited proteolysis. It was found that proteolytic cleavage of flagellins in dissociated flagella into high molecular weight fragments (about 40 000) did not lead to protofilament degradation. It was shown that the most stable fragment is formed from the 60 000 molecular weight flagellin. Cleavage of this fragment led to complete disappearance of protofilaments. On the basis of the data obtained, possible principles of archaebacterial flagellar construction are discussed.Key words: flagellin, archaebacteria, protofilaments,Natronobacterium magadii.