Aeromanas (A) gum, an acidic hetero polysaccharide, in 0.2 M LiCl/dimethyl sulfoxide (DMSO) was fractionated satisfactorily according to the nonsolvent addition method. Eight fractions were chosen to examine their aggregation behavior in aqueous solution. The weight-average molecular weight (M-w), radius of gyration (S-2)(1/2), and intrinsic viscosities [eta] of the fractions in 0.2 M LiCl/DMSO and 0.5 M NaCl aqueous solution at 25 C were measured by static light scattering and viscometry. The results indicated that the A gum was aggregated in 0.5 M NaCl aqueous solution at 25 degreesC, and the aggregates were broken in 0.2 M LiCl/DMSO. The apparent weight-average aggregation number (N-ap) of the fractions increased with the process of fractionation, that is, N-ap increased from 1.1 to 15 with decreasing M-w of the single chain. The fractions obtained by treating with DMSO were more easily dissociated in the aqueous solution, and its N-ap was lower than that of the A gum fractions that were not treated with DMSO. Moreover, the A gum molecules with relatively low M-w aggregated easily to form a compact spherelike structure in the aqueous solution. Elemental analysis and C-13 NMR spectroscopy indicated that DMSO was adsorbed on the A gum molecules caused by the fractionation program; DMSO not only prevented the polysaccharide aggregation but also increased the solubility. A model has been proposed to describe the aggregation behavior of the A gum chains with DMSO overcoat in the aqueous solution.