Naphterpin, a metabolite of Streptomyces aeriouvifer, was shown to be biosynthesized from two units, a polyketide derived naphthoquinone unit and a geranyl side chain. ¹³C-¹³C splitting patterns of naphterpin labeled with [1,2-¹³C₂]acetate indicated the involvement of a symmetric naphthalene intermediate. Although a variety of terpenoidal compounds are frequently produced by fungi and yeasts, it is quite rare that derivatives belonging to the terpene group or possessing a partial terpenoidal structure have been isolated as metabolites of Streptomyces. Such rare examples include pentalenolactone¹), terpentecin²), napyradiomycins³) and furaquinocin⁴) whose biosynthetic origins have been unequivocally revealed by labeling experiments. In this regards, we have been very much interested in the biosynthetic mechanism of naphterpin, a metabolite of Streptomyces aeriouvifer, isolated as an antioxidative agent⁵). Naphterpin is a naphthoquinone metabolite with an additional C10 ring unit of seemingly terpenoidal origin. Its structure has been established as shown in Fig. 1 by NMR and X-ray analyses⁵). In the present report, we describe biosynthetic studies of naphterpin by means of feeding experiments using ¹³C- and ²H-labeled precursors. S. aeriouvifer was cultured as described previously and 24 hrs after inoculation, [1-¹³C] or [1,2-¹³C₂]acetate and [methyl-¹³C]methionine were added at the level of 1 mg/ml and 0.2 mg/ml, respectively. The labeled naphterpins were isolated by solvent extraction followed by silica gel chromatography⁵).