In previous work we have shown that the toxicity of nanomaterials to Daphnia spp. differs with the type of nanoparticle either due to the core of the particle or to the way in which a particle suspension is prepared. The purpose of this study was to investigate the toxicity and antioxidant response of Daphnia pulex in relation to a change in surface functionalization of nanomaterials with the same core material, nC60. Despite the lack of acute toxicity for various nC60 suspensions up to 100 ppm concentration, there was a significant production of the toxicity biomarkers glutathione-S-transferase and catalase, at lower concentrations indicating changes in reactive oxygen species. Nanoparticle functionalization significantly affected this response. Oxidative stress markers appear to be a good predictor of potential future toxicity of nanomaterials. Functionalization alters both toxicity and oxidative stress in whole organism assays.