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A hitherto unknown amino sugar was discovered in lipopolysaccharides isolated from strains of chemotype III of the photosynthetic bacterium Rhodopseudomonas palustris. The sugar (2–3% of the lipopolysaccharides dry weight) is detected as a shoulder on the descending part of the glucosamine peak obtained when acid hydrolysates of the lipopolysaccharides are passed through an amino acid analyzer. It was separated from the other amino sugars of the lipopolysaccharide (i.e. glucosamine, galactosamine, quinovosamine) by high-voltage paper electrophoresis and subsequent paper chromatography. The mass spectrometric analysis of the isolated sugar as deuterium-reduced alditol acetate revealed the presence of a 6-O-methyl-2-amino-2-deoxyhexose. Demethylation of the isolated sugar afforded glucosamine and the demethylation of its ninhydrin degradation product yielded arabinose, which showed that the parental amino sugar is glucosamine. Authentic 6-O-methyl-d-glucosamine exhibited identical properties in paper chromatography and high-voltage electrophoresis as did its alditol acetate derivative in gas-liquid chromatography and mass spectrometry. These data prove that the unknown amino sugar in lipopolysaccharides of Rhodopseudomonas palustris is 6-O-methyl-glucosamine. The strong positive optical rotation of the isolated sugar indicates its d-configuration. 6-O-Methyl-d-glucosamine is part of the O-specific chain and not of thelipid A moiety of the lipopolysaccharide. Periodate-oxidation studies revealed that the sugar, like the other amino sugars of this lipopolysaccharide, does not occupy non-reducing terminal positions to any appreciable extent.