A new nucleoside antibiotic, mildiomycin D, was isolated from the culture broth of Streptoverticillium rimofaciens B-98891as a minor component. Themolecular formula of the antibiotic purified by silica gel and ion exchange resin columnchromatographies was determined to be C19H30N8O8• (2H2O) from its physicochemical data. The ultraviolet and infrared spectra were very similar to those ofmildiomycin, a major component. On the basis of 1H and 13C-NMRspectra and acidic hydrolysates of the compound, the chemical structure of the antibiotic was determined as a deoxy compound at the C8, position in mildiomycin. Mildiomycin D showed weak activities against Gram-positive and negative bacteria, phytopathogenic fungi and some yeasts, and its activity against Rhodotorula rubura was about 40% that of mildiomycin.