<jats:title>Abstract</jats:title><jats:p>A new pteridine compound was isolated from green sulfur photosynthetic bacteria, <jats:italic>Chlorobium limicola f. thiosulfatophilum</jats:italic> NCIB 8327. The structure of this pterin derivative was established to be 1‐<jats:italic>O</jats:italic>‐(<jats:sc>L</jats:sc>‐<jats:italic>erythro</jats:italic>‐5,6,7,8‐ tetrahydropterin‐2′‐yl)‐β‐<jats:italic>N</jats:italic>‐acetylglucosamine (<jats:bold>1</jats:bold>) from <jats:sup>1</jats:sup>H‐NMR and CD spectra as well as from various mass spectrometric techniques and chemical‐cleavage techniques. Upon acid hydrolysis of <jats:bold>1</jats:bold>, equimolar amounts of biopterin (<jats:bold>2</jats:bold>) and <jats:italic>N</jats:italic>‐acetylglucosamine were produced. The structure of the hydrolysis product <jats:bold>2</jats:bold> was confirmed by comparing its NMR, UV, CD, and MS and its chromatographical behavior with those of an authentic specimen. <jats:italic>N</jats:italic>‐Acetylglucosamine was identified by an enzymatic hydrolysis experiment as well as by NMR and thin layer chromatography. Electrospray (ES), fast‐atom‐bombardment (FAB), and thermospray (TS) mass spectrometry of <jats:bold>1</jats:bold> yielded an <jats:italic>M</jats:italic>H<jats:sup>+</jats:sup> at <jats:italic>m/z</jats:italic> 441. Periodate‐oxidation experiments of the intact molecule <jats:bold>1</jats:bold> and of its hydrolysis product <jats:bold>2</jats:bold> are consistent with the proposed structure. Differential I<jats:sub>2</jats:sub> oxidation experiments with the native compound showed that the <jats:italic>in vivo</jats:italic> oxidation state of this pterin is its tetrahydro form. We propose the trivial name ‘limipterin’ for this new compound.