Recently, 2-acetamido-3-O-[(R)- and (S)-1-carboxyethyl]-2-deoxy-D-glucoses (N-acetylmuramic acid and N-acetylisomuramic acid) have been identified as constituents of the O-specific polysaccharides of Yersinia ruckerii II and Proteus penneri 62, respectively. We report now the isolation and identification of 2-acetamido-4-O-[(S)-1-carboxyethyl]-2-deoxy-D-glucose from the O-specific polysaccharide of Proteus penneri 35. The polysaccharide was obtained by mild acid degradation of the lipopolysaccharide isolated from dry bacterial cells, followed by O-deacetylation. Solvolysis with anhydrous HF and anion-exchange HPLC yielded an N-acetylated acidic amino sugar (1). ¹H and ¹³C NMR data, along with nuclear Overhauser effect (NOE) experiments confirming substitution at position 4 of GlcNAc, showed 1 is 2-acetamido-2-deoxyglucose etherified by lactic acid. Authentic (R)- and (S)-2-acetamido-4-O-(1-carboxyethyl)-2-deoxy-D-glucoses were synthesized via alkylation and hydrogenolysis, and comparison revealed 1 matched the (S)-isomer by NMR and specific optical rotation. Thus, the acidic amino sugar is 2-acetamido-4-O-[(S)-1-carboxyethyl]-2-deoxy-D-glucose, a new natural isomer of N-acetylmuramic acid not hitherto found in Nature.