As the worldwide application of silver nanomaterials in commercial products increases every year, and concern about the environmental risks of such nanoparticles also grows. A clear understanding of how different characteristics of nanoparticles contribute in their toxic behavior to organisms are imperative for predicting and control nanotoxicity. Within our research, we investigated the toxic effect of two types of silver nanoparticles (spherical and flat Ag nanoparticles) on zebrafish (Danio rerio) embryos. Particular interest was paid to proper characterization of Ag nanoparticles initially and during the experiment. A proper test medium was found and used for ecotoxicity evaluation. The behavior of flat silver nanoparticles with respect to embryos of zebrafish was analyzed and compared to the ecotoxicity of silver ionic form (AgNO3). Both types of nanoparticles showed a more pronounced toxic effect to Danio rerio embryos than silver ions (AgNO3), while silver nanoplates were more harmful than Ag nanospheres. While previous investigations showed that toxicity of Ag nanoparticles can be explained by the presence of Ag* in solution of silver nanoparticles, our results demonstrate that the harmful effects of nanosilver may be associated with silver nanoparticles themselves than with ionic silver released into solution. (C) 2017 Elsevier B.V. All rights reserved.