We now wish to report the isolation from culture filtrates of an authentic strain of Penicillium cyclopium of a hitherto undescribed metabolic product for which we propose the name cyclopenin. Cyclopenin was isolated in roughly equal amounts from culture filtrates of P. cyclopium grown on either Czapek-Dox or Raulin-Thom solutions by adsorption on charcoal and elution with methanol, with properties including molecular formula C₁₇H₁₄O₃N₂, crystallization from methanol (containing two molecules of water of crystallization, melting at 110-120°C with frothing and water loss, resetting to remelt at 207°C) or ethyl acetate (containing half a molecule of water, melting sharply at 207°C), strong laevorotation in ethanol, presence of a nitrogen-methyl group, and hydrolysis to carbon dioxide, methylamine, and viridicatin. Viridicatin, previously isolated from P. viridicatum, was detected in five other P. cyclopium strains. Its structure as 2:3-dihydroxy-4-phenylquinoline or its keto tautomer was confirmed by oxidative decomposition (yielding o-aminobenzophenone and oxalic acid) and synthesis via cyclization of chloroacetyl o-aminobenzophenone. Based on these results, cyclopenin's structure is proposed as either (VI) or (VII), consistent with its optical activity, hydrolysis products, and chemical properties.