Succinoglycan samples ranging in weight-average molecular weight from 1.4 x 10(5) to 10.5 x 10(5) (in 0.1 M aqueous NaCl at 25 degreesC), prepared from native succinoglycan by sonication, were investigated by static light scattering and viscometory in 0.1 M aqueous NaCl. It is well known that the conformation of the polysaccharide changes at around 65 degreesC. In order to confirm the conformation of the polysaccharide, the property of the polysaccharide in dilute solution was studied at both 25 and 75 degreesC. The molecular weight of a sample at 25 degreesC is the double of the value at 75 degreesC. The molecular weight dependence of (z) and [eta] for succinoglycan shows that the polysaccharide is a rod-like polymer at 25 degreesC and behaves like a semi-flexible polymer at 75 degreesC in 0.1 M aqueous NaCl. Moreover, the linear mass density of the polysaccharide was almost twice that expected for single succinoglycan molecules. These results indicate that the conformation of the polysaccharide would be a double helix at 25 degreesC and the helix melts to a single strand behaving as a semi-flexible chain above 65 degreesC.