The structural organization of bacterial flagella was studied with Bacillus brevis and Escherichia coli MS 1350. The presence of a spherical body at the basis of a flagellum was confirmed. The structural organization of ingredients of the flagellar appratus, its inner and outer part, was investigated. The molecular weight of protein subunits in the filamentous portion of the flagellum was assayed as well as their amino acid composition. The mode of attachment of the flagellum to the cell is discussed.