During a course of the investigation on the biosynthesis of blasticidin S, we isolated a blasticidin S-related metabolite from the strongly basic fraction of the filtered broth of Streptomyces griseochromogenes (a blasticidin S-producing organism). We report the isolation, structure, and biological activity of this compound, demethylblasticidin S. The compound was isolated via a series of chromatographic steps (Amberlite IRC-50, active carbon, Dowex 50W) and recrystallized to analytical purity. Its structure was established by physicochemical properties (molecular formula C₁₆H₂₄O₅N₈·H₂O, melting point 244-248°C dec., optical rotation [α]²⁷D +59°, UV spectra, color reactions, paper chromatography Rf values), ¹H-NMR spectroscopy (lack of the N-methyl singlet present in blasticidin S, confirming the absence of an N-methyl group), and acid hydrolysis (yielding nucleoside cytosinine and new amino acid demethylblastidic acid, characterized via conversion to demethylpseudoblastidone). Addition of ethionine (a transmethylation inhibitor) to the fermentation broth increased demethylblasticidin S yield without considerably affecting blasticidin S production. Biological activity assays showed demethylblasticidin S is almost as active as blasticidin S against rice blast pathogen Piricularia oryzae (MIC 10 μg/ml vs. 5-10 μg/ml for blasticidin S). Greenhouse experiments demonstrated effective protection against rice blast disease (protective values 99.4%-100% at 5-20 μg/ml, comparable to blasticidin S). The oral LD₅₀ in mice was 35 mg/kg, similar to blasticidin S (38 mg/kg).