We have isolated Ilicicolin H (Ia), an antifungal antibiotic from the mycelium of an imperfect fungus, Cylindrocladium ilicicola. Ilicicolin H (Ia) has the molecular formula C₂₇H₃₇NO₄ with physical constants including m.p. 144-150°C, [α]D -17.4°, M⁺ 433, λmax (EtOH) 248 nm (ε 23,200) and 349 nm (ε 5,300), and contains hydroxy groups and a lactam ring (νmax 3590-3400 and 1653 cm⁻¹). Methylation with dimethylsulfate gave a trimethyl derivative (Ib). Alkaline peroxide oxidation yielded p-hydroxybenzoic acid, indicating a p-hydroxyphenyl substituent. Hydrogenation with 10% Pd-C afforded a tetrahydro derivative (II), while PtO₂ hydrogenation produced three products (III, IV, V). Acid hydrolysis of III (C₂₇H₄₁NO₅) gave lactam VI (C₁₁H₁₅NO₂, a 4-hydroxypyridone-2 derivative) and carboxylic acid VII (C₁₄H₂₆O₂). Spectral data (¹H NMR, IR, UV) and deuterium exchange experiments suggested III has a β-diketone system and is a 3-acyl-4-hydroxy-pyridone-2 derivative, leading to the conclusion that Ia has a conjugated system identical to tenellin (a metabolite of Beauveria sp.). Further studies on VII, including oxidation, epimerization, and Baeyer-Villiger reaction, identified (+)-menthanecarboxylic acid, confirming the absolute configuration of VII. Detailed 100-MHz ¹H NMR double- and triple-resonance experiments on Ia and its derivatives (monoepoxide XX, monobromide XXI) clarified the absolute configuration of Ilicicolin H (Ia).