Fifteen fractions of aeromonas gum, a heteropolysaccharide produced by the strain Aeromonas nichidenii, have been studied by static light scattering and viscometry with dimethylsulfoxide containing 0.2 M LiCl at 25degreesC as the solvent. Data for the z-average radius of gyration and the intrinsic viscosity covering a molecular weight range from 4.5 x 10(5) to 2 x 10(6) show the polymer to behave like a semiflexible chain in this solvent, and are analyzed on the basis of the wormlike chain by coarse-graining the heteropolysaccharide molecule. It is shown that these data and those for the particle scattering function are consistently explained by this model with a (mean) persistence length of 10 (+/- 1) nm and a (mean) linear mass density of 1450 (+/- 100) nm(-1), and that the heteropolysaccharide chain is as stiff as cellulose derivatives.