Aurantiamine, a new substituted diketopiperazine, has been isolated from Penicillium aurantiogriseum. The gross structure was established mainly by spectroscopy and the configuration at the sole chiral centre by comparison with the isomeric viridamine and by using biosynthetic arguments. Aurantiamine is produced by the majority of strains of P. aurantiogriseum var. aurantiogriseum and var. neoechinulatum and it allows a chemotaxonomic separation between some morphologically closely related varieties in P. aurantiogriseum.