Inhibition of photosynthetic electron transport in isolated spinach (Spinacea oleracea L.) thylakoids and competitive binding to spinach, redroot pigweed (Amaranthus retroflexus L.), and potato (Solanum tuberosum L.) thylakoids were investigated for sorgoleone (2-hydroxy-5- methoxy-3- [(8'Z, 11'Z)-8',11',14'-pentadecatriene]-p-hydroquinone) in comparison with other photosystem II inhibitors. Sorgoleone inhibited photosynthetic electron transport as effectively as DCMU (diuron) [N'(3,4- dichlorophenyl)- N,N-dimethylurea]. Rapid inhibition was observed by both compounds at concentrations of less than 0.2 μM. Sorgoleone competed with atrazine [6- chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] for binding to spinach, triazine-susceptible redroot pigweed or potato thylakoids as effectively as metribuzin [4-amino-6-(1,1-dimethylethyl)-3- (methylthio)- 1,2,4-triazine-5(4H)-one] or diuron. No competition was observed for bentazon [3-(methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2- dioxide]. Double reciprocal plots in the presence or absence of 0.5 μM sorgoleone showed that sorgoleone is a competitive inhibitor of atrazine. Competitive binding was not observed in atrazine-resistant redroot pigweed or potato thylakoids. We conclude that sorgoleone is a potent inhibitor of photosynthetic electron transport and binds to the same Q(B) niche of the D1 protein as diuron, atrazine, or metribuzin.