Nine samples of gellan gum in the sodium form, ranging in weight-average molar mass from 3.47 x 10(4) to 1.15 x 10(5) at 40degreesC, were investigated by static and dynamic light scattering and viscometry in 25 mM aqueous NaCl both at 40 and at 25degreesC. The ratios of the molar mass at 25degreesC (in the ordered state) to that at 40degreesC (in the disordered state) were in the range of 1.99 to 2.07, supporting the scheme of the conformational transition of gellan gum between a disassociated single chain and an associated chain composed of two molecules. Focusing on the effects of polydispersity, the intrinsic viscosities, radii of gyration, and hydrodynamic radii were analyzed on the basis of unperturbed wormlike chain models. The persistence lengths were evaluated as 9.4 nm at 40degreesC and 98 nm at 25degreesC.