The polyketide origin of fungal anthraquinones has been firmly established by the isotopic tracer studies with acetate and malonate as the precursors. On our experiments, it has been found that ^<14>C-emodin and ^<14>C-emodin-anthrone were efficiently incorporated into the anthraquinonoids of P. brun-neum and P. islandicum, and that ^3H-(-)flavoskyrin into skyrin, (-)rugulosin and (-)rubroskyrin. On the contrary, the metabolites other than emodin were recovered unchanged and the incorporations into the metabolites other than themselves were insignificant. The efficient incorporation of aromatic precursors reveals that the second-ary alcoholic group of the modified anthraquinones must be formed by the hydrogenation of aromatic ring and the incorporation of (-)flavoskyrin into the dimeric compounds suggests that the mechanism of the dimerization to afford modified anthraquinones involves (π2s+π4s) cycloaddition of Diels-Alder type and not phenol oxidative coupling. From these results, the biosynthetic pathway of anthraquinonoids of Penicillium species is proposed as Fig.3.