Interactions with environmental parameters may alter the ecotoxicity of nanoparticles. The present study therefore assessed the (in)direct effects of nanoparticulate titanium dioxide (nano-TiO(2)) towards Gammarus fossarum, considering nano-TiO(2)'s photocatalytic properties at ambient UV-intensities. Gammarids' habitat selection was investigated using its feeding preference on leaf discs either exposed to or protected from UV-irradiation in presence of nano-TiO(2) as proxy (n = 49). UV-irradiational one induced a significant preference for UV-protected habitats, which was more pronounced in simultaneous presence of nano-TiO(2). This behaviour may be mainly explained by the UV-induced formation of reactive oxygen species (ROS) by nano-TiO(2). Besides their direct toxicity, ROS may have lowered the leaf-quality in UV-exposed areas contributing (approximately 30%) to the observed behavioural pattern. Since the predicted no effect concentration of nano-TiO(2) in combination with UV irradiation falls below the predicted environmental concentration this study underpins the importance of considering environmental parameters during the risk assessment of nanoparticles.