A number of studies have been published concerning the potential ecotoxicological risks of titanium dioxide nanoparticles (nano-TiO2), but the results still remain inconclusive. The characteristics of the diverse types of nano-TiO2 must be considered in order to establish experimental models to study their toxicity. TiO2 has important photocatalytic properties, and its photoactivation occurs in the ultraviolet (UV) range. The aim of this study was to investigate the toxicity of nano-TiO2 to indicators organisms of freshwater and saline aquatic systems, under different illumination conditions (visible light, with or without UV light). Daphnia similis and Artemia salina were co-exposed to a sublethal dose of UV light and different concentrations of nano-TiO2 in the form of anatase (TA) or an anatase/rutile mixture (TM). Both products were considered practically non-toxic under visible light to D. similis and A. salina (EC50(48h) > 100 mg/L). Exposure to nano-TiO2 under visible and UV light enhanced the toxicity of both products. In the case of D. similis, TM was more toxic than TA, showing values of EC50(48h) = 60.16 and 750.55 mg/L, respectively. A. salina was more sensitive than D. similis, with EC50(48h) = 4 mg/L for both products. Measurements were made of the growth rates of exposed organisms, together with biomarkers of oxidative stress and metabolism. The results showed that the effects of nano-TiO2 depended on the organism, exposure time, crystal phase, and illumination conditions, and emphasized the need for a full characterization of nanoparticles and their behavior when studying nanotoxicity.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação para o Desenvolvimento da UNESP (FUNDUNESP)State Univ Campinas UNICAMP, Inst Biol, Dept Biochem, BR-13083862 Campinas, SP, BrazilEMBRAPA, Lab Ecotoxicol & Biosafety, BR-13820000 Jaguariuna, SP, BrazilSao Paulo State Univ UNESP, Dept Environm Engn, BR-18087180 Sorocaba, SP, BrazilSao Paulo State Univ UNESP, Dept Environm Engn, BR-18087180 Sorocaba, SP, BrazilFAPESP: 10/06226-2