A survey of the s-triazine herbicide 2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine, a constituent of antifouling paints, was conducted in the coastal waters and estuaries of Kent, Sussex and Hampshire, UK. The compound was detected at significant levels in areas of high boating activity, particularly in marinas and in the Hamble Estuary, Hampshire. In contrast, lower levels of the triazine herbicides, simazine and atrazine, were observed at these marine sites.