Résumé : This paper presents a novel assay that allows a quick and robust assessment of the effects of biocides on the initial settling and establishment of marine photoautotrophic biofilms. The assay integrates the response of the multitude of indigenous fouling organisms, which overcomes a major limitation of existing assays which are largely limited to testing only lab-cultivatable species. The assay was evaluated using eight antifouling biocides, for which full concentration-response curves are presented. The efficacy ranking, based on EC 98 values from most to least efficacious compound is: copper pyrithione >TPBP >DCOIT >tolylfluanid>zinc pyrithione >medetomidine >copper (Cu 2+ ), while the ecotoxicological ranking (based on E C10 values) is irgarol, copper pyrithione >zinc pyrithione >TPBP >tolylfluanid >DCOIT >copper (Cu 2+ ) > medetomidine. The algaecide irgarol did not cause full inhibition. Instead the inhibition leveled out at 95% effect at 30 nmol l -1 , a concentration that was clearly lower than for any other of the tested biocides.