Cadmium sulfide (CdS) quantum dots have a number of current applications in electronics and solar cells and significant futurepotential in medicine. The aim of the present study was to examine the toxic effects of CdS quantum dots on the marine clamScrobicularia plana exposed for 14 d to these nanomaterials (10 mg Cd L–1) in natural seawater and to compare them with soluble Cd.Measurement of labile Cd released from CdS quantum dots showed that 52% of CdS quantum dots remained in the nanoparticulate form.Clams accumulated the same levels of Cd regardless of the form in which it was delivered (soluble Cd vs CdS quantum dots). However,significant changes in biochemical responses were observed in clams exposed to CdS quantum dots compared with soluble Cd. Increasedactivities of catalase and glutathione-S-transferase were significantly higher in clams exposed in seawater to Cd as the nanoparticulateversus the soluble form, suggesting a specific nano effect. The behavior of S. plana in sediment showed impairments of foot movementsonly in the case of exposure to CdS quantum dots. The results show that oxidative stress and behavior biomarkers are sensitive predictorsof CdS quantum dots toxicity in S. plana. Such responses, appearing well before changes might occur at the population level, demonstratethe usefulness of this model species and type of biomarker in the assessment of nanoparticle contamination in estuarine ecosystems.