Many sessile suspension-feeding marine organisms rely on chemical defense to keep their surfaces free from fouling organisms. The brominated cyclopeptides barettin (cyclo[(6-bromo-8-entryptophan)arginine]) ( 1) and 8,9-dihydrobarettin (cyclo[(6-bromotryptophan)arginine]) ( 2) from the cold-water sponge Geodia barretti have previously displayed settlement inhibition of barnacle larvae in a dose-dependent manner. In this paper, we describe a novel dibrominated cyclopeptide, bromobenzisoxazolone barettin (cyclo[(6-bromo-8-(6-bromobenzioxazol-3(1 H)-one)-8-hydroxy)tryptophan)]arginine) ( 3), which we have isolated from G. barretti and which displays settlement inhibition of barnacle larvae ( Balanus improvisus) with an EC 50 value of 15 nM. The chemical structure was determined using MS and 2D-NMR.