The feeding deterrent activity of methanol extract from the brown alga Dilophus okamurai was examined on the sea urchin Strongylocentrotus nudus by a bioassay with a cellulose plate. Two cubebane-type diterpene alcohols obtained from the neutral fraction of the extract were the most active compounds. Spata-13, 17-dien-10-ol, one of the two spatane-type diterpenes with an inhibitory effect on the settlement and metamorphosis of the abalone veligers, exhibited moderate activity. These results shed light on part of the chemical defense of D. okamurai against herbivores.