Disclosed is a composition comprising a combination of 4-bromo-2-(4-chlorophenyl)-5- (trifluoromethyl)-1H-pyrrole-3-carbonitrile (tralopyril), or a salt thereof, as component (I), and as a component (II) a copper compound selected from Cu2O, Cu(OH)2, copper pyrithione, CuSCN, CuCO3; whereby component (I) and one of the components (II) are in respective proportions to provide a synergistic effect against fouling organisms.