Although the field of prokaryotic cell biology is well-advanced now, mycoplasmas were the first bacteria in which the existence of a cytoskeleton was postulated. Despite this head-start, the cytoskeletons of mycoplasmas are presently less well understood than those of other bacteria. This deficit is principally attributable to three factors: the novel nature of most of the cytoskeletal elements as compared with other bacteria, which have the advantage of being related to eukaryotic cytoskeletal proteins; differences among the cytoskeletons of different mycoplasma species; and the fastidiousness of mycoplasmas, which complicates efforts to perform protein biochemistry. In better studied mycoplasmas like Mycoplasma pneumoniae, a major component of the cytoskeleton is associated with the attachment organelle, a polar structure that is essential for adherence to host cells, involved in gliding motility, and associated with cell division. Mycoplasma mobile also has structures that appear to be involved in gliding motility, though in contrast to the structures of M. pneumoniae, these are extracellular. Some other species also have distinct subcellular structures.