Sixteen narrow-distribution samples of sodium poly(styrenesulfonate) (NaPSS) ranging in weight-average molecular weight M-w from 2.6 x 10(3) to 6.5 x 10(5) were prepared by the radical polymerization of sodium-p-styrenesulfonate, followed by repeated fractional precipitation, and were studied by static light scattering, sedimentation equilibrium, and viscometry in 0.5 M aqueous NaCl at 25 degrees C and in 4.17 M aqueous NaCl at 16.4 degrees C, the Theta point, where the light scattering second virial coefficient vanished for high M-w. The characteristic ratio (i.e., the mean-square radius of gyration at the Theta point divided by the degree of polymerization) for high-molecular-weight NaPSS was found to be considerably smaller than that for polystyrene in cyclohexane. Data for the intrinsic viscosity [eta] in 4.17 M aqueous NaCl at Theta were analyzed by the theory of Yoshizaki et al. (Macromolecules 1988, 21, 165) for the wormlike chain, a special limit of the helical wormlike chain, to obtain 0.23 (+/- 0.03), 0.69 (+/- 0.07), and 1.2 (+/-0.1) nm for the monomeric contour length, the persistence length q, and the chain diameter, respectively. This q value is much smaller than the intrinsic persistence length of 1.2 nm widely used in the literature for experimental tests of available theories for the electrostatic persistence length. At the lower NaCl concentration of 0.5 M where intramolecular excluded-volume effects were significant, the molecular weight dependence of [eta] was explained quantitatively by a combination of Yoshizaki et al.＇s theory and the quasi-two-parameter theory with a larger q of 1.6 nm and an excluded-volume strength parameter of 1.6 nm.