Treponema phagedenis, Treponema vincentii, and Treponema refringens were studied in the electron microscope by means of negative staining and sectioning techniques. The length and width of the cells were within the same limits for all three strains examined. Statistical analysis, however, demonstrated a difference in the wavelengths of the cells. Organisms of T. phagedenis and T. vincentii had blunt ends and possessed cell wall surface layers in which a regular substructure was only occasionally revealed, while cells of T. refringens had tapered ends and a distinctly structured cell wall surface layer. Generally, cells of the strains investigated had 4–6 flagella inserted at each end. Two bundles of flagella, one from each end of the cell, wound around the organism and overlapped in the middle of the cell. The flagella with their insertion organelles were identical for the three strains studied. Two bundles of cytoplasmic tubules were detected in the interior of the treponemes after treatment with either sodium deoxycholate or Myxobacter AL-1 protease 1. The two bundles of cytoplasmic tubules overlapped in the middle of the cell. The results of the present morphological study are compared with results obtained by other investigators studying other characteristics of these strains and it is concluded that the three strains studied belong to three different species.