The studies of lipopolysaccharides (LPS) of eight Rahnella aquatilis strains, isolated from different sources have shown that they contain both S- and R-types of molecules. This fact is evidenced by the presence of high-molecular fraction of O-specific polysaccharides (O-PS) and low-molecular fraction of core oligosaccharides. The predominant monosaccharides of core oligosaccharides were glucose, galactose. The presence of only one high-molecular fraction O-PS was a characteristic feature of all investigated strains. The predominant monosaccharides of O-PS were galactose, glucose, mannose, rhamnose and fucose. 3-hydroxytetradecanoic (48.9-93.1%), dodecanoic (2.9-12.1%), tetradecanoic (4.1-25.3%) and hexadecanoic (2.8-15.3%) acids have been obtained in lipids A of LPS depending on the strain. The presence in lipid A of R. aquatilis of only 3-hydroxytetradecanoic acid which is characteristic of Enterobacteriaceae proves the correct ascribing of the isolated strains to this family.