Based on 13C- and 'H-NMR spectral analysis the structures of miharamycins A and B have been determined to be novel 9-substituted 2-aminopurine nucleoside antibiotics as shown in Fig. 3. Miharamycins A and B are antibiotics produced by Streptomyces miharaensis SF-489 and active against rice blast disease. Although these compounds were isolated some 15 years ago, their structures have remained unclear, because chemical manipulations to prepare suitable derivatives for structural elucidation were unsuccessful. We wish to report herein the structural studies of these novel nucleoside antibiotics, miharamycins A and B accomplished based on 'H- and 13C-NMR spectral analysis.