In the course of screening for biologically active substances among fungal metabolites, a new compound with antibacterial activity was isolated from Aspergillus zonatus IFO 8817 and designated as aszonapyrone A (C₂₈H₄₀O₅). This study reports the isolation, structural elucidation, and antibacterial activity of this compound. Structural analysis using IR, UV, MS, ¹H-NMR, and ¹³C-NMR spectroscopy, combined with chemical reactions (e.g., OsO₄ oxidation, alkaline hydrogen peroxide oxidation), revealed aszonapyrone A contains a tricyclic diterpene skeleton, a 3'-substituted-4'-hydroxy-6'-methyl α-pyrone moiety, and an acetoxyl group. Aszonapyrone A showed a minimum inhibitory concentration (M.I.C.) of 6.3 µg/ml against Staphylococcus aureus 209 P and S. aureus res.