Thermoplasma acidophilum has no cell wall, and so its irregular shape implies the presence of a cytoskeleton. When soluble extracts of T. acidophilum were incubated in vitro they increased in viscosity, suggestive of a polymerizable component. Optimal conditions for the viscosity increase coincided with physiological ionic concentrations. Electron micrographs of negatively stained extracts showed a meshlike lattice of elements 10 nm in diameter similar to nuclear lamins. However, immunologically there was no cross-reaction with lamins nor with the other eukaryotic cytoskeletal proteins tested: tubulin, calmodulin, giardin, actin or myosin.