The thermoreversible gelation of agarose in different water/dimethyl sulfoxide (DMSO) mixtures has been studied by polarimetry and small-angle neutron scattering, and the modulus-concentration relations have been determined. In the sol state it has been found that the chain is fairly rigid, independent of the solvent composition, with a persistence length exceeding 9 nm. Theoretical fits of the scattering patterns are consistent with conformations close to single helices. The mass per unit length, which decreases with increasing DMSO content, is accounted for by considering the formation of an agarose/DMSO complex. The mechanical properties of the gel state depend little upon the solvent composition. The notion of double helices in the gel state is discussed in the light of these results.