This study reports the first accurate measurements of the flexural rigidity of microtubules and actin filaments, which are crucial components of the eukaryotic cytoskeleton. The researchers analyzed the thermal fluctuations in the shape of these filaments to estimate their flexural rigidity. For taxol-stabilized microtubules, the mean flexural rigidity was found to be \(2.2 \times 10^{-23} \, \text{Nm}^2\) (with 6.4% uncertainty) for seven unlabeled microtubules and \(2.1 \times 10^{-23} \, \text{Nm}^2\) (with 4.7% uncertainty) for eight rhodamine-labeled microtubules. These values are similar to earlier estimates obtained by modeling flagellar motion. For rhodamine-phalloidin-labeled actin filaments, the flexural rigidity was measured to be \(7.3 \times 10^{-26} \, \text{Nm}^2\) (with 6% uncertainty), consistent with previous results. The flexural rigidity of microtubules corresponds to a persistence length of 5,200 μm, indicating that they are rigid over cellular dimensions. In contrast, the persistence length of an actin filament is only about 177 μm, which may explain why actin filaments within cells are usually cross-linked into bundles. The greater flexural rigidity of microtubules compared to actin filaments is attributed to the larger cross-section of microtubules. If tubulin were homogeneous and isotropic, the Young's modulus of microtubules would be approximately 1.2 GPa, similar to Plexiglas and rigid plastics. The results suggest that microtubules are almost inextensible, with cell compliance primarily arising from filament bending or sliding between filaments rather than stretching.This study reports the first accurate measurements of the flexural rigidity of microtubules and actin filaments, which are crucial components of the eukaryotic cytoskeleton. The researchers analyzed the thermal fluctuations in the shape of these filaments to estimate their flexural rigidity. For taxol-stabilized microtubules, the mean flexural rigidity was found to be \(2.2 \times 10^{-23} \, \text{Nm}^2\) (with 6.4% uncertainty) for seven unlabeled microtubules and \(2.1 \times 10^{-23} \, \text{Nm}^2\) (with 4.7% uncertainty) for eight rhodamine-labeled microtubules. These values are similar to earlier estimates obtained by modeling flagellar motion. For rhodamine-phalloidin-labeled actin filaments, the flexural rigidity was measured to be \(7.3 \times 10^{-26} \, \text{Nm}^2\) (with 6% uncertainty), consistent with previous results. The flexural rigidity of microtubules corresponds to a persistence length of 5,200 μm, indicating that they are rigid over cellular dimensions. In contrast, the persistence length of an actin filament is only about 177 μm, which may explain why actin filaments within cells are usually cross-linked into bundles. The greater flexural rigidity of microtubules compared to actin filaments is attributed to the larger cross-section of microtubules. If tubulin were homogeneous and isotropic, the Young's modulus of microtubules would be approximately 1.2 GPa, similar to Plexiglas and rigid plastics. The results suggest that microtubules are almost inextensible, with cell compliance primarily arising from filament bending or sliding between filaments rather than stretching.