Nanoscale zerovalent iron (nZVI)-based nanotechnologies are increasingly being used for environmental remediation; however, the fate and ecotoxicologic effects of nZVI remain unclear. Larvae of medaka fish (Oryzias latipes) underwent 3-14 days' aqueous exposure to thoroughly characterized solutions containing carboxymethyl cellulose (CMC)-stabilized nZVI, bare nZVI, nanoscale iron oxide (nFe(3)O(4)) or ferrous ion [Fe(II)(aq)] at μg/L-mg/L levels to assess the causal toxic effect(s) of iron nanoparticles (NPs). Acute larval mortality was decreased in the order of Fe(II)(aq) > CMC-nZVI > nZVI > nFe(3)O(4). CMC-nZVI (100 mg/L) increased hypoxia and reactive oxygen species (ROS) and Fe(II)(aq) production, thus increasing mortality and oxidative stress response as compared with unstabilized nZVI. Additionally, nFe(3)O(4) and nZVI were more bioavailable than suspended CMC-nZVI or Fe(II)(aq). Antioxidant activities were significantly altered by induced intracellular ROS levels in larvae with subchronic exposure to nFe(3)O(4) or Fe(II)(aq) at environmentally relevant concentrations (0.5-5 mg/L). We report on different organizational biomarkers used for rapidly assessing the lethal and sublethal toxicity of nZVI and its stabilized or oxidized products. The toxicity results implicate a potential ecotoxicological fate and impact of nZVI on the aquatic environment.