A lipopolysaccharide (LPS) from Budvicia aquatica DRL 20186 was isolated, studied, and chemically identified. It was shown to be lowly toxic, but highly pyrogenic. Its fatty acid composition was similar to that of the LPS from other Enterobacteriaceae, with predominance of tetradecanoic (32.7%) and 3-hydroxytetradecanoic acids (23.8%). Hexadecenoic (20.4%), hexadecanoic (11.8%), and dodecanoic acids (8.4%) were also revealed. Double immunodiffusion in agar by the Ouchterlony method revealed antigenic activity of the B. aquatica DLR 20186 LPS in a homologous system. In cross reactions, however, it did not interact with the antisera to other @B. aquatica@ strains.