Acidic polysaccharides were isolated from the Pseudomonas aeruginosa II (Sandvik) and V (Verder-Evans) lipopolysaccharides on mild acid hydrolysis followed by gel filtration on Sephadex G-50. The Sandvik II polysaccharide consists of 2-acetamido-2-deoxy-D-galacturonic acid, 2-acetamido-2,6-dideoxy-D-glucose, and L-rhamnose in the ratio 1:1:2. The Verder-Evans V polysaccharide contained the same monosaccharides and, in addition, a D-glucose residue. On the basis of 13C NMR data, methylation analysis, Smith degradation and solvolysis with hydrogen fluoride, the following structures were determined for the repeating units of the polysaccharides: (Formula: see text).