Polydora websterii, a harmful lugworm that has serious adverse effects on pearl oyster cultivation, was inhibited by a marine diterpene, crenulacetal C, isolated from the brown alga Dictyota dichotoma, Based on consideration of the activity-structure relationship, several synthetic compounds having an aromatic moiety with a hydroxyalkyl chain were prepared. Bioassay using larvae of Polydora websterii as well as pearl oysters (Pinctada fucata martensii) suggested that 1-(2-furyl)-1-nonanol was the most promising inhibitor.