We have studied the dynamics of F-actin of different average filament lengths by polarized and depolarized dynamic light scattering. The experimental data have been compared with model calculations for rigid and semiflexible macromolecules. The polarized DLS spectra follow the predictions for semiflexible rods with a persistence length of (7.5 +/- 2.5) mu m. Because there is no theory available that includes flexibility in the calculation of the depolarized dynamic structure factor, to a first approximation we used the diffusion coefficients for semiflexible molecules in our rigid-rod model calculations. In the dilute solution regime, this modification provided good agreement between experimental and calculated depolarized correlation functions for F-actin with an average degree of polymerization (X) less than or equal to 350. Internal bending motions give significant contributions to the scattered intensity only for longer filaments. Concentration-dependent depolarized dynamic light scattering measurements on actin filaments revealed a slow relaxation mode in semidilute solution. A comparison with the corresponding polarized DLS measurements is presented.