A strain of Streptomyces isolated from a soil sample collected in Jiangsu Province, China, was found to produce a new antibiotic selectively active against Xanthomonas sp. The strain has not yet been identified but the antibiotic was designated as xanthostatin because of its selective activity against Xanthomonas sp. Fermentation was carried out at 28°C for 96hr in a jar fermentor containing 18 liters of a medium composed of glucose (2%), soluble starch (1%), meat extract (0.1%), dry yeast (0.4%), soybean flour (2.5%), NaCl (0.2%) and K2HPO4 (0.005%). The filtered broth (25 liters) was extracted with ethyl acetate at pH 4, and the extracts were concentrated to dryness, giving 4.8g of an oily residue that was crystallized from methanol. The crystals were collected by filtration, washed with methanol (400mg), and further purified by preparative HPLC (Nucleosil 5C18, 20mm×250mm; solvent, methanol-H2O=8:2; speed, 6.0ml/min), affording 150mg of pure crystals of the antibiotic. Xanthostatin was colorless crystals with a melting point of 157~160°C with decomposition, optically active ([α]j^ = -76° (c 0.5, MeOH)), soluble in methanol, ethyl acetate and chloroform but hardly soluble in benzene, hexane and water. It was a neutral compound with no titrable group, stable in neutral and acidic conditions but unstable in alkaline solution. From elementary analysis and FD mass spectroscopy [M+ 895, (M+Na)+ 918], the molecular formula was established as C44H61N7O13·H2O (Calcd.: C, 57.83; H, 6.90; N, 10.73; O, 24.53; Found: C, 58.14; H, 6.77; N, 10.70; O, 24.39). Its UV spectrum (λmax nm (E1%): 230 (90), 265 sh (7.3), 268 sh (8.1), 274 (8.5), 282 (7.1)) was characteristic of tyrosine, unchanged in acidic and alkaline methanol. The IR spectrum showed main absorption bands at 3300, 2950, 1725, 1630, 1525, 1500, 1450, 1400, 1295, 1245, 1195, 1080, 1050, 1020, 825, 745 and 695 cm-1, indicating ester and amide bonds. The 1H NMR spectrum gave positive Rydon-Smith and Dragendorf tests but negative ninhydrin test. Amino acid analysis of the hydrolysate (6N HCl, 120°C, 16hr) showed threonine, serine, glycine and tyrosine in a molar ratio of ~2:1:1:0.6, plus two unidentified amino acids. Xanthostatin was specifically inhibitory to Xanthomonas oryzae and Xanthomonas citri, with no effect on other tested bacteria and fungi. Mice tolerated an intraperitoneal injection of 400mg/kg. Although ascamycin and xanthocidin are known to selectively inhibit Xanthomonas sp., no peptide antibiotic with such selectivity was previously reported.