From the fermentation products of a Streptomyces culture number Y-83,40369, we have isolated naphthomycin A (1)1,2) as a minor constituent and a new compound designated naphthomycin H (2); a light yellow crystalline substance analyzed as C39H44CINO9, mp 150°C (dec) as the major constituent. Naphthomycin H (2) is shown to be a geometrical isomer of naphthomycin B (3)3) by extensive 1H NMR double-resonance studies.