The structure of the novel nucleoside antibiotic A201A has been determined by a combination of chemical and spectroscopic methods. It is composed of 6-dimethylaminopurine, 3-amino-3-deoxyribose, p-hydroxy-alpha-methylcinnamic acid, a novel unsaturated hexofuranose and 3,4-di-O-methylrhamnose. Structures have also been assigned to several related minor factors simultaneously isolated from the fermentation broth. These unique nucleosides have very interesting similarities and differences in structure with the known antibiotics puromycin and hygromycin A.