In this study, using a bioassay-guided isolation and purification procedure, we obtained 3-chloro-2,5-dihydroxybenzyl alcohol from a marine-derived Ampelomyces species that effectively inhibited larval settlement of the tubeworm Hydroides elegans and of cyprids of the barnacle Balanus amphitrite. The inhibitive effect on larval settlement was nontoxic and the EC50 of 3-chloro-2,5-dihydroxybenzyl alcohol ranged from 3.19 microg ml-1 to 3.81 microg ml-1 while the LC50 was 266.68 microg ml-1 for B. amphitrite cyprids; EC50 ranged from 0.67 microg ml-1 to 0.78 microg ml-1, and LC50 was 2.64 microg ml-1 for competent larvae of H. elegans, indicating that inhibitive effect of this compound was nontoxic. At a concentration of 50 mug per disc, this compound showed strong inhibitive effects on the growth of 13 out of 15 marine bacterial species tested in disc diffusion bioassay. Overall, the high inhibitory activities against bacteria and larval settlement as well as the non- or low-toxic nature of this compound to the barnacle and polychaete larvae suggest this compound could be a potent antifoulant and/or antibiotic.