Three new sesquiterpenoids (1- 3) with a dihydro-beta-agarofuran skeleton were isolated from Celastrus vulcanicola. Their structures were elucidated on the basis of spectroscopic analysis, including homonuclear and heteronuclear correlation NMR experiments (COSY, ROESY, HSQC, and HMBC), and the absolute configurations were determined by circular dichroism and chemical correlations. Their effects on photosynthesis were tested. Sesquiterpene 1 (50 microM) inhibits both light-dependent synthesis of ATP and the electron flow in chloroplasts, whereas at high concentrations the electron flow inhibition was partially reversed. Therefore, 1 behaves as a Hill reaction inhibitor and a weak energy transfer inhibitor and has two targets of interaction: one located at the oxygen-evolving complex, and the other located at the light-activated Mg (2+)-ATPase. Compound 2 was inactive, whereas 3 acts with the same mechanisms as 1 but was less active. Celastrus vulcanicola J. Donnell Smith (Celastraceae) is a subtropical woody vine distributed in Central America and the Caribbean. Its chemical constituents and biological activity have not yet been investigated.