A new antibiotic mildiomycin, strongly active against powdery mildew, was isolated from the culture filtrate of Streptoverticillium rimofaciens B-98891. It is a water-soluble basic antibiotic and was purified by ion-exchange and adsorption chromatography. The molecular formula of the purified compound was determined to be C19H30N8O9(H2O) from physical and chemical data. The UV and NMR spectra suggested that this antibiotic is a nucleoside. On acidic hydrolysis it gave 5-hydroxymethyl cytosine which has not previously been found in nucleoside antibiotics.