<jats:p>2‐Amino‐2‐deoxyhexuronic acid was found to constitute the main sugar component of the specific part of <jats:italic>Shigella sonnei</jats:italic> phase I lipopolysaccharide. The sugar was isolated from the lipopolysaccharide by acid hydrolysis and purified on a Dowex 50×8 column. From the chemical reactivity, gas chromatography‐mass spectrometry, paper chromatography, high‐voltage electrophoresis and deamination experiments it has been concluded that the isolated compound is a 2‐amino‐2‐deoxyhexuronic acid.