Details of the separation of the metabolites (I-V) of Chaetomium thielavioideum are presented. The structure of a new mycotoxin named chaetochromin was studied by physical method (ultraviolet, infrared, 1H- and 13C-nuclear magnetic resonance (NMR), and mass spectrum) and it was identified as the bis(naphthodihydropyran-4-one (II). The assignment of the 13C-NMR signals was largely achieved with the aid of the two- and threebond spin-spin couplings between 13C and 1H.