Under controlled levels of fragmentation by sonication in the presence of ATP, actin filaments reached different length distributions corresponding to different steady states of polymerization. When the sonication was interrupted, a spontaneous decrease in the number of filaments and corresponding increase in length was observed. The kinetics of the process of length redistribution subsequent to fragmentation was studied for the simpler case of F-actin filaments at equilibrium in the presence of ADP. Analysis of the data shows that a diffusion-like random walk mechanism of length redistribution quantitatively accounts for the observations much better than the previously proposed end-to-end reannealing of filaments. The involvement of this process in the kinetics of actin polymerization and in establishing the length distribution at steady state is discussed.