Acute (96 h) and chronic (21 d) exposures of Daphnia magna neonates were carried out with nano- and micro-sized Ag and CeO2 particles to assess the influence of both material and size of particles on mortality and moulting. Mortality rates for silver in the acute exposures were: AgNP, 56.7 +/- 23.3% at 0.1 mg L-1 and 100 +/- 20% at 1 mg L-1, and micro-Ag, 13.3 +/- 6.7% at 0.1 mg L-1 and 80 +/- 20% at 1 mg L-1. CeO2 was not acutely toxic at concentrations up to 10 mg L-1. Mortality for Ag over 21d at concentrations of up to 0.05 mg L-1 was low, while mortality of 30% was observed for 0.001 mg L-1 of nano-Ag. CeO2, with the exception of the 10 mg L-1 of nano-CeO2 (100% mortality by day 7), was non-toxic. Inhibition of moulting and growth in the acute study occurred at toxic concentrations (Ag particles), and at 10 mg L-1 of nano-CeO2. The chronic study revealed reduced moulting at 0.001 mg L-1 of nano-Ag and 0.01 and 0.05 mg L-1 of both sizes of Ag, but there was no impact on D. magna size, and no effects of CeO2. The toxicity of nano-CeO2 may be attributed to reduced feeding and physical interference with the daphnids' carapace, resulting in reduced swimming ability. Our results suggest that Ag NPs in particular have the potential to be harmful to aquatic invertebrates after release into the environment, whereas CeO2 particles appear to cause little adverse effects, and only at environmentally irrelevant concentrations.