In contrast to vast number of terpenoid compounds produced by plants and fungi (more than 22,000), Streptomyces produce only small number of terpenoid compounds. Among them, naphterpin1), furaquinocins2), napyradiomycins3) and terpentecin4) were proved to be biosynthesized via the classical mevalonate pathway. Very curiously, however, pentalenolactone which is produced by several Streptomyces species, is synthesized via the non-mevalonate pathway5) which has recently been proposed by ROHMER et al.6,7). In this pathway (see Fig. 1), glyceraldehyde 3-phosphate and pyruvic acid condense to form 1-deoxyxylulose 5-phosphate which is then converted to isopentenyl diphosphate (IPP) via 2-C-methyl-D-erythritol 4-phosphate. In addition to pentalenolactone, we have reported in a previous paper that the dimethylally side chain of carquinostatin, an antioxidative substance produced by Streptomyces exfoliatus, was biosynthesized via the same nonmevalonate pathway8). These results surprisingly imply that IPP, the building unit of terpenoids, is formed by the mevalonate pathway in some Streptomyces species, while it is formed by a completely different pathway, i. e., the non-mevalonate pathway in other Streptomyces species. The question thus occurred to us was that "Are Streptomyces with the mevalonate pathway taxonomically different from those possessing the non-mevalonate pathway?" As a further step to pursue this interesting phenomenon, we chose novobiocin as a metabolite of Streptomyces niveus9) with a dimethylally side chain unit (Fig. 1), which was reasonably assumed to be originated from IPP, and studied its biosynthesis by the use of 13C-labeled precursors. Biosynthetic studies using 14C-precursors revealed that the aminocoumarin and benzoic acid moieties of novobiocin were derived from tyrosine10) and that the sugar unit was formed from glucose11) with the introduction of a methyl group to either C-6" or C-7" from the methyl group of methionine12). The origin of the C5 side chain at C-3, however, remained to be clarified. Analysis of the 13C-NMR spectrum of novobiocin labeled with sodium [1-13C] acetate showed no incorporation of the precursor into C-7 and C-9 excluding the operation of the mevalonate pathway for the formation of IPP in this organism (data not shown). On the other hand, addition of [1-13C] glucose into the fermentation broth increased the signal intensities of C-7 and C-10 in the dimethylally side chain by 1.4 to 2.2 times (Table 1 and Fig. 2). This labeling pattern is explained by incorporation of glucose via the non-mevalonate pathway to novobiocin after metabolism to dihydroxyacetone phosphate and glyceraldehyde 3 phosphate by glycolysis14) (Fig. 1). The incorporation of [1-13C] glucose to the side chain was apparently improved by addition of the precursor at a later stage showing sole operation of the nonmevalonate pathway throughout the fermentation period. This result is in sharp contrast to the case of naphterpin, where the non-mevalonate pathway was used for the production of IPP at the earlier fermentation stage with gradual replacement by the mevalonate pathway at the later fermentation stage15). Thus it is concluded that S. niveus possesses only the nonmevalonate pathway for the formation of IPP. This is the third example in which Streptomyces utilizes the non-mevalonate pathway for the formation of IPP. As far as our data are concerned, common Streptomyces seem to possess only the non-mevalonate pathway. Further studies on the biosynthesis of terpenoid compounds produced by Streptomyces are now under way to uncover the relationship between the ability to utilize the mevalonate pathway by Streptomyces and their taxonomical properties.