Intrinsic viscosities for sodium poly(styrenesulfonate) in aqueous sodium chloride at 25°C have been determined for 15 samples ranging in weight-average molecular weight from 3.8×103 to 6.5×105 at five salt concentrations Cs between 0.05 and 2M. Their molecular weight dependence at each Cs is fairly satisfactorily explained by the theory of Yoshizaki et al. for unperturbed wormlike chains combined with the quasi-two-parameter theory for excluded-volume effects. The estimated persistence length q and excluded-volume strength B both increase with decreasing Cs. This increase in q is not quantitatively described by the known theories for the electrostatic persistence length when the previously determined q of 0.69nm in 4.17M aqueous NaCl at the theta point is used for the intrinsic persistence length. It is. also shown that the values of B computed on the conventional bead model and the rodlike segment model in the Debye—Hueckel approximation with the ion condensation hypothesis are too large compared to the experimental estimates at lower Cs of 0.05 and 0.1M though the latter model gives considerably smaller B than does the former one.