Evidence has accumulated recently that not only eukaryotes but also bacteria can have a cytoskeleton. We used cryo–electron tomography to study the three-dimensional structure of Spiroplasma melliferum cells in a close-to-native state at ∼4-nanometer resolution. We showed that these cells possess two types of filaments arranged in three parallel ribbons underneath the cell membrane. These two filamentous structures are built of the fibril protein and possibly the actin-like protein MreB. On the basis of our structural data, we could model the motility modes of these cells and explain how helical Mollicutes can propel themselves by means of coordinated length changes of their cytoskeletal ribbons.