The lipopolysaccharide of strain Pseudomonas fluorescens IMV 1433 (biovar I) was isolated and investigated. 3-Hydroxydecanoic, 2-hydroxydodecanoic, 3-hydroxydodecanoic, dodecanoic, hexadecanoic, hexadecenoic, and octadecenoic acids were identified in the composition of lipid A. The total amount of hydroxy acids makes up 60%. In the hydrolysate of lipid A, components of the were revealed hydrophilic skeleton--glucosamine and phosphoethanolamine--were revealed. Glucose, galactose, rhamnose, glucosamine, galactosamine, alanine, phosphoethanolamine, phosphorus, and 2-keto-3-deoxyoctulosonic acid were identified in the core of the lipopolysaccharide molecule. O-specific polysaccharide chains of the lipopolysaccharide are built from repeating trisaccharide units consisting of the residues of 2-acetamido-2,6-dideoxy-L-glucose, 2-acetamido-2,6-dideoxy-D-glucose and 4-acetamido-4,6-dideoxy-D-galactose. The lipopolysaccharide studied is similar to the lipopolysaccharide of P. fluorescens IMV 1152 (biovar I) in the structure of lipid A and core oligosaccharide, and identical to it in the structure of the O-chain (this identity correlates with the high level of the serological relationship of the strains).