In a galactose-isoleucine medium at 37 "c, Streptomyces venezuelae ISP5230 produces jadomycin (I), the first 8H-benz[h]oxazolo[3,2-f]phenanthridine derivative to be isolated from natural sources. The structure of 1 was assigned by interpretation of the spectral data. Cultures of Streptomyces venezuelae ISP52302 in a glucose-isoleucine medium at 28 "C produce chloramphenicol; when glucose is replaced by galactose and the temperature is raised to 37 'C, the cultures produce a novel group of related pigmented antibiotics. We describe herein the structural determination of jadomycin (l), one of these antibiotics. The structures of the remaining metabolites will be reported elsewhere.