Inhibition of the Fts family of proteins causes the growth of long filamentous cells, indicating that they play some role in cell division. FtsZ polymerizes into protofilaments and assembles into the Z-ring at the future site of the septum of cell division. We analyze the rigidity of GTP-bound FtsZ protofilaments by using cryoelectron microscopy to sample their bending fluctuations. We find that the FtsZ-GTP filament rigidity is κ=4.7±1.0×10 -27 Nm 2, with a corresponding thermal persistence length of lp=1.15±0.25 μm, much higher than previous estimates. In conjunction with other model studies, our new higher estimate for FtsZ rigidity suggests that contraction of the Z-ring may generate sufficient force to facilitate cell division. The good agreement between the measured mode amplitudes and that predicted by equipartition of energy supports our use of a simple mechanical model for FtsZ fibers. The study also provides evidence that the fibers have no intrinsic global or local curvatures, such as might be caused by partial hydrolysis of the GTP.