Lipopolysaccharide was isolated from the cell-walls of a human strain of Aeromonas hydrophila by the aqueous phenol method in 0.58% yield (based on dry weight of bacteria). The lipopolysaccharide consisted of SR-polysaccharide, core-oligosaccharide and lipid A; there was no O-specific polysaccharide. The core had the composition D-galactose, D-glucose, D-glycero-D-manno-heptose, L-glycero-D-manno-heptose and D-glucosamine in a molar ratio of 1:1:2:4:1. Glucosamine was linked to an L-glycero-D-manno-heptose residue by a bond which was resistant to hydrolysis. The D-glucosamine-(1----7)-LD-heptose disaccharide was isolated and identified by the mass spectrum of its methylated alditol and the heptose residue not observed under normal hydrolysis conditions was easily determined after deamination of the complete core. Methylation analysis, chemical degradation, periodate and chromium trioxide oxidations and nuclear magnetic resonance (13C and 1H NMR) spectroscopy were used to identify the structure of the core oligosaccharide as: (formula: see text)