The antibiotic As1A, strongly inhibitory to Phytophthora capsici Leonian in vitro and in vivo, was isolated from the broth culture of Streptomyces libani Baldacci & Grein using various chromatographic procedures. The molecular formula of the antibiotic As1A was deduced to be C45H74O11 (M+H, m/z 791.5307) by high resolution fast atom bombardment - mass spectroscopy. The analysis of 1H-NMR (nuclear magnetic resonance) and 13C-NMR spectroscopy, DEPT experiment, and two-dimensional NMR spectral data revealed that the antibiotic is a macrolide antibiotic having a 26-membered alpha, beta-unsaturated macrolactone ring with a conjugated diene fused to a bicyclic spiroketal. Based on the comparison of NMR data and other chemical properties, the antibiotic As1A turned out to have the same structure as oligomycin A. The antibiotic As1A showed a high level of inhibitory activity against Botrytis cinerea Pers.: Pers., Cladosporium cucumerinum Ellis & Arthur, Colletotrichum lagenarium (C.P. Robin) Berkhout, Magnaporthe grisea (Herb.) Barr, and P. capsici, ranging from 3 to 5 µg·mL-1 of MICs. However, no antimicrobial activity was found against yeasts and bacteria. In further evaluation under greenhouse conditions, developments of the Phytophthora disease, anthracnose, and leaf blast were markedly inhibited on pepper (Capsicum annuum L. cv. Hanbyul), cucumber (Cucumis sativus L. cv. Baekrokdadaki), and rice (Oryza sativa L. cv. Nakdong) plants by treatments with the antibiotic As1A, respectively. Control efficacies of the antibiotic As1A against these plant diseases were in general similar to those of metalaxyl, chlorothalonil, and tricyclazole. The antibiotic As1A did not show any phytotoxicity on pepper, cucumber, and rice plants even at 500 µg·mL-1.Key words: Streptomyces libani, oligomycin A, antifungal activity, plant disease control.