Nine acetyl hemiacetals, (+)-ovalifoliene (1), (+)-ovalifolienal (2), (+)-ovalifolienalone (3), deacetoxyovalifoliene (6), (+)-ovalimethoxy l (7), (+)-ovalimethoxy ll (8), (+)-plagiochiline A (9), (+)-plagiochiline B (10), and (+)-9-acetoxyovalifoline (11), which possess the ent-2,3-secoalloaromadendrane skeleton and display plant-growth-inhibitory properties, and their bicyclic precursors, (–)-hanegokedial (4) and (+)-hanegoketrial (5), have been isolated from the methanol extract of Plagiochila semidecurrens together with the related enantiomeric sesquiterpenoids (12)–(15a). The structures and absolute configurations have been determined on the basis of chemical and spectroscopic evidence and are coincident with the result obtained by X-ray analysis of (+)-ovalifolienalone (3). The biological activity is also described.