A commercial sample of xanthan was (i) sonicated and fractionated by GPC on Sepharose C1-2B with 0.1 M NaCl as eluant/solvent. The angular dependence of scattered light was measured (offline) and interpreted in terms of the wormlike chain model using the "master curve procedure". The persistence length was estimated as L(P) approximate to 150 nm. The mass per unit length M(L) approximate to 2000 g/mol . nm indicates double-stranded chains. (ii) The parent sample was studied at various ionic strengths I (0.01 mol/L less than or equal to I less than or equal to 0.3 mol/L). The conditions for optimum solubility and clarification (ultracentrifugation/membrane filtration) were checked. At higher ionic strength, the sample was only partly soluble and the solution contained supermolecular structures even after clarification. Good solubility was observed in 10 mM NaCl. The Zimm procedure was used to obtain the average molar mass M(W), the second virial coefficient, and the radius of gyration R(g). The interpretation of the scattering function led to M(W) = 2.88 x 10(6) g/mol, R(g) = 241 nm, L(P) = 149 nm, a polydispersity sigma(M) = 0.5, and double-stranded chains. At room temperature, solutions revealed a tendency for aggregation, which could be prevented by storage at elevated temperature.