Today nanoparticles (NPs) have many applications in commercial products due to their small size and peculiar properties that, conversely, make them potentially toxic for humans and the environment. ZnO NPs are largely used in many personal care products, such as sunscreens and cosmetics. In this study the cytotoxic effects of ZnO particles with different sizes (ZnO Bulk, >100nm; ZnO NPs, 100nm and ZnO NPs, 14nm) upon the first developmental stages of the sea urchin Paracentrotus lividus, are evaluated. Morphological alterations are also assessed by embryotoxicity tests. The cytogenetic analysis highlighted that ZnO NPs interfere with cell cycle inducing a dose-dependent decrease of mitotic activity and chromosomal aberrations at higher concentrations (30μM). Moreover, the larval development was affected by ZnO NPs 100nm (EC50=0.46 [0.30-0.63] μM [Zn]) in a dose-dependent way. Size-dependent toxicity was instead not obtained for ZnO NPs. From our results could be highlighted that the presence of embryos, blocked in pre-larval stage, could be due to the induction of chromosome aberrations by ZnO particles, confirming that cytogenetic analyses allow evaluating possible NPs action mechanisms.