In the course of studies on the biosynthesis of blasticidin S, a nucleoside antibiotic, we have isolated six novel cytosine nucleosides, named pentopyranines A to F, from the fermentation broth of Streptomyces griseochromogenes. The structural relationship between these nucleosides and cytosinine (the nucleoside moiety of blasticidin S) implied the intermediacy of cytosine nucleosides with a hexuronic acid moiety in the biosynthesis of blasticidin S. Accordingly, we succeeded in isolating a cytosine nucleoside designated pentopyranic acid (I) from the amphoteric fraction of the fermentation broth of S. griseochromogenes. Through analyses including UV, 1H-nmr, mass, CD spectra, and comparison with an authentic sample, the structure of I was determined to be 1-(β-D-glucopyranosyluronic acid)-cytosine. The isolation of I is taken as evidence that it is the direct precursor of the hypothetical intermediate X in blasticidin S biosynthesis and that the oxidation of the hydroxymethyl group of the hexose moiety takes place prior to that at C-4.