A novel antibacterial compound, macrocarpal A, was isolated from the leaves of Eucalyptus macrocarpa, and its structure was determined on the basis of an X-ray crystal structure analysis. Macrocarpal A is composed of a phloroglucinol dialdehyde and diterpene, having a 3-membered ring, a 5-membered ring and a 7-membered ring. Plants of the Eucalyptus species are widely distributed throughout the world and famous for their rapid growth. Recently, several biologically active compounds have been isolated from Eucalyptus species as biological resources. Grandinol¹) is a rooting inhibitor and was isolated from adult tissue of E. grandis. We have also isolated and identified grandinol as an antibacterial compound from E. perriniana.²) Euglobals³⁻⁵) have been isolated from the buds and leaves of E. globulus and were found to show anti-inflammatory activity. Robustadials⁶) are antimalarial compounds and were isolated from E. robusta. We are interested in antimicrobial compounds from plants of the Eucalyptus species, and have reported antibacterial compounds from E. perriniana.²) In this paper, we described the isolation and structural elucidation of a novel antibacterial compound from E. macrocarpa. It was named macrocarpal A (1, Fig. 1) after the species name of the plant.