Welan, in aqueous solution, has "weak gel" properties analogous to those of ordered xanthan but, unlike xanthan, shows no evidence of conformational change between 0 and 100 degrees C. When the polymer is dissolved in dimethyl sulphoxide (Me2SO) rather than in water, however, there is a massive decrease in viscosity and total loss of gel-like character. In mixtures of the two solvents, the change in rheology occurs over a narrow range of composition (approximately 85-90% v/v Me2SO for 0.5% welan). On heating and cooling in a solvent close to the lower end of the critical range (86% Me2SO), the polymer shows typical order-disorder and disorder-order transitions [as monitored by optical rotation, differential scanning calorimetry, and temperature-course of rheological change]. When solutions of disordered welan in Me2SO are poured into excess water they form cohesive strings of gel. We interpret these results as showing that: (1) the stable conformation of welan in water is the double helix structure identified by X-ray fibre diffraction in the solid state; (2) in native welan, as biosynthesised, the strands are perfectly paired, and ordered along their full length; (3) on exposure to high concentrations of Me2SO, the native structure is dissociated into disordered coils; (4) rapid renaturation from the disordered state gives shorter helices, with exchange of partners to form a stable cross-linked network.