6-Hydroxy-5-isopropyl-3,8-dimethyl-4a,5,6,7,8,8a-hexahydronaphthalen-2(1H)-one (HHO) is an allelopathic substance that is related to allelochemical metabolism in Eupatorium adenophorum. In this study, the full-length complementary DNA of two phenylalanine ammonia-lyase (PAL) genes and a fragment of the cinnamate 4-hydroxylase (CA4H) gene from E.?adenophorum (Ea) were cloned. The expression of all three genes was increased by the HHO treatment, with varying sensitivity: EaPAL2 >EaPAL1 >EaCA4H. Southern blotting suggested that there is a third member of the PAL gene family in this species.