The behaviour of polyglutamic acid and of its alkali salts in aqueous electrolyte solutions of varying ionic strengths has been examined by light scattering and in the ultracentrifuge, and parallel diffusion and viscosity studies have been made. Both behave in solution as flexible chain molecules of effective volume large for their weight. The effective volume per mole is much greater for the ionized form than for the largely unionized form (free acid), and the effective volume of the alkali salts increases markedly with reduction in the ionic strength of the solvent as would be expected from their polyelectrolyte nature. Sedimentation studies indicate a substantial degree of ion-pair formation between the polyglutamate ion and the cation of the solvent electrolyte. Despite this, the alkali polyglutamates show a 'secondary charge effect' of considerable magnitude, much greater than that predicted from the measured electrophoretic mobility of the polyglutamate ion. In contrast to the free acid, both the sedimentation and diffusion coefficients are markedly concentration-dependent. These coefficients have been extrapolated to infinite dilution, the sedimentation coefficients on the assumption that their inverse varies linearly with concentration and the diffusion coefficients by the method of Mandelkern & Flory (1951), using a value of the second virial coefficient derived from light-scattering data. Combination of these extrapolated values leads to a molecular weight for the free acid of 172000 and for sodium polyglutamate of 200000 $\pm $ 5000 after allowance for the effects of selective solvation. Detailed analysis of the sedimentation data for sodium polyglutamate in salt solutions of varying density indicates that the molecule is selectively solvated with not more than its own weight of water, a more probable figure being approximately 60%. Detailed light-scattering studies of sodium polyglutamate have been made at ionic strengths 0$\cdot $2 and 1$\cdot $1. The mean of a number of determinations gives a weight-average molecular weight of 238000 and a number-average of 88000, the latter figure being in good agreement with a value obtained by assay of free amino groups. Values of the root-mean-square end-to-end distance of coils having the number- and z-average molecular weights are also given.