Increasing use of metallic nanomaterials is likely to result in release of these particles into aquatic environments; nevertheless it is unclear whether these materials present a hazard to aquatic organisms. The impact of contaminated sediment containing 14-nm gold nanoparticles (AuNPs) was investigated in the zebrafish Danio rerio exposed for 20 days to two concentrations, 16 and 55 µg/g dry weight. AuNPs were released from the sediment to the water column, and during this period the mean concentrations of AuNP in the filtered water fraction were 0.25 ± 0.05 and 0.8 ± 0.1 µg/L, respectively. A similar experiment with ionic gold contamination was simultaneously performed to obtain a positive control. AuNP exposure triggered various effects in fish tissues including modifications of genome composition, shown using a random amplified polymorphic DNA-PCR genotoxicity test. Expression of genes involved in oxidative stress, mitochondrial metabolism, detoxification and DNA repair were also modulated in response to AuNP contamination. Gold altered neurotransmission, since brain acetylcholine esterase activity increased for both tested doses of AuNP but not for ionic gold. Gold accumulation in fish tissues demonstrated the lower bioavailability of AuNP compared to ionic Au, and underlined the higher toxic potential of the nanoparticle form.Read less <