The diffusion behavior of the flexible highly charged polyelectrolyte sodium poly(styrenesulfonate) has been studied extensively with dynamic light scattering in the dilute concentration regime. The apparent diffusion coefficient was determined for one molar mass and five ionic strengths 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 a wormlike chain model, including the persistence length, the excluded volume, and counterion condensation. A satisfactory agreement was found. From the polyelectrolyte concentration dependence of the apparent diffusion coefficient, the diffusion second virial coefficient could be obtained. The ionic strength dependence of the diffusion second virial coefficient k(d) was found to be linearly proportional to 1/c(s), in agreement with the prediction of small ion-polyion coupled mode theories.