The basal body of Wolinella succinogenes consists of a central rod, a set of two rings (L and P rings), a basal disk from 70 to 200 nm in diameter, and a terminal knob. In negatively stained preparations of flagellar hook-basal body complexes, some disks remain fixed perpendicularly to the grid and show that such a disk is located on the distal side of the P ring. The basal disks have been isolated with and without the P ring; in both cases there is a hole in the center of the disk. The diameter of the disk is smaller in the presence of the P ring. The L-P ring complex is therefore assumed to be a bushing for the rod. Thin sections of whole bacteria and spheroplasts reveal that the disk is attached to the inner surface of the outer membrane. At the insertions of the flagellar hook-basal body-basal disk complexes, depressions are visible in negatively stained preparations of whole bacteria and spheroplasts. A new ringlike structure is connected to an elongation of the basal body into the cytoplasm in both preparations. Its diameter (60 nm) is larger than that of the M ring. A heavily stained compartment can be seen in between the new ringlike structure and the basal disk, which may be formed by the energy transducing units.