The ecotoxicological assessment of alternative "booster" biocides is urgently needed in order to develop environmentally acceptable antifouling paints. However, research has focused mainly on single compounds, and there is still a lack of data on their mixture toxicity. The present study investigated the single and mixture toxicity of three of the most widely used antifouling biocides: zinc pyrithione, chlorothalonil and Sea-Nine, using the sea-urchin (Paracentrotus lividus) embryo-larval bioassay. Also, the predictive ability of the concentration addition (CA) and independent action (IA) concepts for antifouling mixtures was evaluated. Both concepts failed to accurately predict the toxicity of the antifouling mixtures, with the exception of the zinc pyrithione and Sea-Nine mixture, which was accurately predicted by the IA concept, suggesting a dissimilar mode of action of those substances. In general, CA predicted consistently higher toxicity than IA; however, CA overestimated the toxicity of the studied mixtures by a factor of only 1.6, representing a reasonable worst-case approach to be used in the predictive hazard assessment of antifouling mixtures. Finally, the present study demonstrates that the risk of antifouling mixtures for the early developmental stages of sea urchin is higher than the risk of each single substance, and therefore, the inclusion of mixture considerations in the development of water quality criteria for antifouling compounds is strongly recommended.