Novel highly viscous polysaccharides composed mainly of (a) glucose, (b) galactose, (c) mannose, and (d) glucuronic acid, the molar ratio of (a):(b):(c):(d) being 10:3-6:0.5-2:0.5-2 (preferably 10:3-4:1-2:1-2). The invention also provides the process for producing the highly viscous polysaccharides which comprises cultivating a highly viscous polysaccharide-producing microorganism which is an acetic acid bacteria in a nutrient medium until a substantial amount of polysaccharides has accumulated in the nutrient medium, and recovering the accumulated polysaccharides from the nutrient medium.