Technical Abstract: Our study of the mechanism(s) by which sorgoleone (1) acts as a photosystem II (PS II) inhibitor led to the isolation of a new benzoquinone derivative, 2-hydroxy-5-ethoxy-3-[(Z,Z)-8',11',14'- pentadecatriene]-r-benzoquinone (2),from the root exudate of sorghum [Sorghum bicolor (L.) Moench (Poaceae)]. The structure of 2, which is being given the name 5-ethoxy-sorgoleone, was determined by spectroscopic means. A methoxy derivative (3) of 1 was also prepared. Both 2 and 3 caused a reduction in oxygen evolution by thylakoid membranes and induced variable chlorophyll fluorescence. However, these compounds were less active inhibitors of PS II than 1. These observations indicate that modifying the 3-hydroxy and 5-methoxy substituents of 1 affects its inhibitory activity.