The natural quinone, hydroxydietrichequinone (3-heptadec-8-enyl-2-hydroxy-5-methoxy-[1,4]benzoquinone) is a secondary metabolite of Cyperus javanicus. We found that this quinone inhibited both mitochondrial respiration and photosynthesis in their electron transportation systems. The quinone was found to have a mode of action against the ubiquinone reductase site from the results of different electron donor experiments on intact mitochondria from rat liver. The electron transport system, photosystem-II (PS-II), in chroloplast from spinach leaves was inhibited by the quinone in a similar way to that of the triazin sires herbicide, atrazin, with its mode of action against PS II. This natural quinone has a long aliphatic chain (C17) including an unsaturated bond at its midpoint. We recognized 8-9 unsaturated bonds in the aliphatic chain from an MS analysis of the methylthio-addact, and spectral data presumed a configuration of cis. form.