An antimicrobial compound was isolated from the MeOH extract of Catalpa ovata G.$D_{ON}$ fruits, and its structure was Identified as 4,9-dihydroxy-2,2-dimethyl-3,4-Uihydronaphtho[2,3-b]pyran-5,10-dione (HMNP). The antimicrobial activity of the Un was determined by measuring the dose-response inhibiton of microbial growth in liquid cultures and then compared with that of lapachol, a well known antimicrobial 1,4-naphthoquinone. The antimicrobial activity of the HMNP was more effective than that of lapachol over a wide range of test organisms. Gram-positive bacteria, yeast, and fungi ($IC_{50}$ $20-75muM$) were found to be more sensitive to the HMNP than Cram-negative bacteria ($IC_{50}$ > $100muM$). The HMNP also inhibited germination of spores of many fungi. The morphological defurmation of the fungal spores was induced by the treatment of HMNP, as illustrated by Scanning Electron Microscopy (SEM).