The in vivo responses of C. dubia to nanoparticles exemplified by a photoactive titanium oxide (TiO₂) and a non-photocatalytic aluminum oxide (Al₂O₃) were studied. Both nanomaterials inhibited the growth of C. dubia at concentrations ca. >100mg/L. The EC50 value was 42 and 45mg/L in the presence of TiO₂ and Al₂O₃, respectively, based on 3-brood reproduction assay. Results implied that reactive oxygen species (ROS) may not be totally responsible for the adverse effects exerted on the invertebrate. Aggregation and interaction among nanoparticles, C. dubia, and algal cells, major food source of Daphnia, played a significant role on the responses of C. dubia to nanoparticles. Dynamic energy budget (DEB) analysis was used to assess the impact of nanoparticles on the energy allocation of C. dubia. Results indicated that nanoparticles could disrupt the assimilation and consumption of energy in C. dubia dramatically. The assimilation energy was negatively correlated to the concentration of nanomaterials, a reduction from 11 to near 0μg-C/animal/day in the presence of TiO₂ or Al₂O₃ nanoparticles at a nanoparticle concentration of 200mg/L. The energy consumed for life-maintenance increased also with increase in the concentration of nanomaterials. Results clearly demonstrated the importance of energy disruption in determining the toxicity of nanoparticles toward C. dubia.