Bioassay-guided purification of a methanol extract from Amomum tsao-ko led to the isolation of the bicyclic nonane isotsaokoin (1) as the major active principle, an isomer of the previously reported tsaokoin (2). The stereochemical relationship of 1 and 2 was investigated by NOE experiments and Mosher ester derivatization. Compound 1 showed antifungal activity against Trycophyton mentagrophytes.