FtsZ, a prokaryotic homolog of eukaryotic tubulin, is a major constituent of the bacterial Z-ring, which contracts the cell wall during cell division. Because the mechanical properties of FtsZ are unknown, its function in the maintenance and constriction of the Z-ring is not well understood. Here, quantitative rheornetry shows that, at physiological concentrations, FtsZ filaments form, extremely rapidly, highly elastic networks within physiological time scales (similar to minutes), much faster than other major dynamic cytoskeletal filaments, including microtubule, actin, and vimentin in eukaryotes. FtsZ networks display a relatively low viscosity and a high resilience against shear stresses, as well as an elasticity that depends weakly on concentration, G similar to C-0.57, a power-law dependence consistent with crosslinked flexible filaments. Calcium, whose intracellular concentration increases during bacterial division, further enhances the elasticity of FtsZ networks through filament bundling, an effect that occurs in the presence of GTP, not GDP. These studies suggest that FtsZ filaments have the toughness to provide strong mechanical support for the maintenance and circumferential constriction of the bacterial Z-ring. (c) 2005 Elsevier Inc. All rights reserved.