Flagella isolated from Treponema pallidum Nichols after treatment with the detergents Teepol and sodium deoxycholate, and with the enzyme Myxobacter AL-1 protease 1, were studied in the electron microscope after negative staining. The flagella were found to consist of 1) a sheathed shaft, 2) a hook with a honey-combed substructure, 3) a narrow collar with a smooth appearance, and 4) a basal knob which seemed to consist of a single disc. Comparison was made with descriptions of flagella isolated from other spirochetes, and with flagella isolated from gram-negative bacteria. Treatment of Treponema pallidum Nichols with sodium deoxycholate or with the protease 1 (AL-1 enzyme) revealed two bundles of intracytoplasmic microtubules, one bundle at each end of the organism. Each bundle consisted of 6–8 microtubules and each tubule had a diameter of 7–7.5 nm. The possible taxonomic significance of the presence of microtubules in some species of Treponema is discussed.