The new cyclic peptide antibiotic, radamycin (1) and the known thiopeptide methylsulfomycin I (2) have been isolated from the fermentation broth of a Streptomyces sp. RSP9. The structure of radamycin was elucidated by NMR, LC-MS and FAB-MS and was established as a thiopeptide with oxazole and thiazole moieties, and several unusual amino acids. Radamycin is a thiopeptide antibiotic structurally related to a known family of antibiotics whose members include berminamycins1,2), sulfomycins3-5), promothiocins6), and A10255 complex7). Members of this family characteristically possess a cyclic peptide core composed mostly of thiazole rings, oxazole rings, several unusual dehydroamino acids and an unique pyridine ring structure. This report describes the physico-chemical properties and structure determination of a new thiopeptide radamycin (1) which was produced by Streptomyces sp. RSP9.