AbstractA methanol extract of leaves of oat seedlings grown in sand cultures in the dark contained a compound which inhibited the growth ofOphiobolus graminis.The inhibitory factor was isolated and proved to be present in the plant as methoxyhydroquinone glucoside. The glucoside was readily hydrolysed to the corresponding aglucone. The methoxyhydroquinone, or possibly its oxydation product, methoxy‐P‐benzoquinone, was inhibitory to bothOphiobolus graminisvar.graminisandOphiobolus graminisvar.avenae, whereasFusarmm oxysporumvar.lycopcrsiciwas not affected. Synthetic methoxyhydroquinone at 80 mg/l gave a 100% inhibition ofOphiobolus graminisvar.graminis.After being exposed to 80 mg/l of the inhibitor for 24 h the mycelium was unable to initiate growth when transferred to a fresh nutrient solution. Only extracts from young leaves showed inhibitory activity, extracts from mature leaves giving no inhibition. The hydroquinone, or its glucoside, was not detected in roots of young seedlings, where avenacin was the only antifungal compound pres