The inhibitory effects of five pseudoguaninolide sesquiterpenes—neoambrosin, damcinic acid, damsin, ambrosin and hymenin—isolated from the aerial parts of Ambrosia maritima were evaluated on seed germination and seedling growth of wild oat, Avena fatua L. The tested sesquiterpenes caused pronounced reduction in seed germination, particularly at the higher concentrations of 1 and 2 mM. Damsin was the most potent compound with seed germination percentage of 16.7% at 2 mM. The results of seedling growth test showed that the sesquiterpenes are potent inhibitors of root growth and ambrosin and damsin were the most potent compounds with EC50 values of 0.22 and 0.24 mM, respectively. Similarly, strong inhibition of shoot growth was observed with ambrosin (EC50 ¼ 0.30) being the most active compound. Nevertheless, the inhibition of root growth by all compounds was greater than that of shoot growth. Interestingly, the five tested sesquiterpenes showed greater inhibitory effect on germination, root and shoot growth than a reference herbicide, imazamethabenz.