In this study dynamic light scattering experiments have been performed on the flexible highly charged polyelectrolyte sodium poly(styrenesulfonate). The apparent diffusion coefficient was determined for five molar masses at a fixed ionic strength of 0.1 M at dilute polyelectrolyte concentrations. The apparent diffusion coefficients extrapolated to zero polyelectrolyte concentration were compared with theoretical Do values calculated with a theoretical procedure based on the translational friction coefficient of the wormlike chain model of Yamakawa and Fujii. With the excluded volume theories of Fixman and Skolnick, and Barrett, the electrostatic persistence length according to Le Bret, and the counterion condensation theory of Manning, a satisfactory agreement was found for all molar masses. From the polyelectrolyte concentration dependence of the apparent diffusion coefficient, the diffusion second virial coefficient could be obtained. The molar mass dependence of the diffusion second virial coefficient was found to be linearly proportional to the molar mass and could be interpreted with small ion-polyion coupled mode theories.