The relative biodegradability in seawater of a number of compounds in current use in antifouling paints viz. Sea‐Nine™ 211 antifoulant (4,5‐dichloro‐2‐n‐octyl‐4‐isothiazolin‐3‐one), IrgarolR1051 (2‐methylthio‐4‐tert‐butylamino‐6‐cyclopropylamino‐s‐triazine), diuron (3‐(3,4‐dichlorophenyl)l‐l‐dimethylurea), chlorothalonil (tetrachloroisophthalonitrile) and TBTO (tributyltin oxide) was investigated. The disappearance of the each compound from seawater was monitored over 8 w by bioassay using the ship‐fouling diatomAmphora coffeaeformis. The results show, that under the test conditions employed, biodegradability ranges from very readily biodegradable (Sea‐Nine 211) to non‐biodegradable (diuron and Irgarol 1051). The results are discussed in relation to published data on biocide degradation.