A series of 5-aryl-1,3,4-oxadiazole-2-thiols was found to inhibit trans-cinnamate 4-hydroxylase (C4H) from Populus kitakamiensis, which was expressed in yeast. 5-Phenyl-1,3,4-oxadiazole-2-thiol showed inhibitory activity comparable to 2-hydroxy-1-naphthoic acid, a known C4H inhibitor. Studies on the structure-activity relationship indicated that the presence of a thiol group was significant for stronger activity. Of the compounds tested, 5-(3-fluorophenyl)-1,3,4-oxadiazole-2-thiol was the most active. (C) Pesticide Science Society of Japan.