A series of 10 intra-laboratory bioassays and an inter-laboratory assay involving nine laboratories, were performed to validate the use of Amphibalanus amphitrite larvae as test organisms for ecotoxicological studies. The standardization protocol utilized Cadmium Nitrate as a reference toxicant and larval immobilization (after 24 and 48 h) as the end-point. The statistical data analysis showed high homogeneity in intra-laboratory EC(50) values among bioassay repetitions (coefficient of variation (CV)=15.8% after 24 h and 16.9% after 48 h); also CV values obtained in inter laboratory comparison exercise were not higher than the 50%, variation coefficient mentioned in the ISO standards for the precision of inter-laboratory assays. The results demonstrated that the protocol implemented for the balanus larval bioassay comply with the international standards both for intra- and inter-laboratory precision.