<jats:p>A major pterin was isolated by reverse‐phase high‐performance liquid chromatography from cellular extract of vegetative cells of <jats:italic>Dictyostelium discoideum</jats:italic> after perchloric deproteinization and oxidation with acidic iodine. This compound was characterized by its chromatographic behavior, its absorption and fluorescence properties, by its oxidation product with alkaline permanganate, by secondary ion mass spectrometry and by circular dichroism. The final identification was obtained by comparison with authentic materials. It is concluded that the major pterin of <jats:italic>D. discoideum</jats:italic> is the compound 6‐(D‐<jats:italic>threo</jats:italic>‐1,2‐dihydroxypropyl)‐pterin. The name dictyopterin is proposed for this new natural isomer of L‐biopterin.