TreponemaReiter was studied by negative staining and ultrathin sectioning techniques after treatment with the detergents, Teepol and sodium deoxycholate, and with the proteolytic enzyme AL‐1. Special attention was paid to the insertion ends of the endoflagella, and the structures revealed have been compared with those published by other workers on the attachment ends of bacterial flagella of various species. All the treatments freed the flagella from the treponeme though the basal discs were often obscured by membrane fragments adhering to them. The endoflagella were found to consist of the following structural parts: 1) a sheathed shaft, 2) a hook that differs from the shaft both in width and in substructure, 3) a narrow, smooth collar that connects the hook to the basal disc, and 4) a single basal disc of about 270 Å in diameter. In cells treated with sodium deoxycholate and with AL‐1 enzyme, a bundle of 6–8 microtubules was observed to commence or to terminate close to the flagellar basal discs at each end of the cells. The two bundles of microtubules overlap in the middle of the organism. In sectioned cells these microtubules were seen in the cytoplasm close to the inner layer of the cytoplasmic membrane and always directly under the place where the endoflagella were found in the interspace between this membrane and the cell wall. Each individual microtubule showed a diameter of about 75 Å both in negatively stained and in sectioned m