Silver nanoparticles (AgNP) are now widely used as antibacterial products, and their potential toxicities in aquatic organisms are a matter of increasing concern. In the present study, we conducted experiments to reveal the acute and chronic toxicities of AgNP and its bioaccumulation from both aqueous and dietary sources in a model freshwater cladoceran, Daphnia magna. No mortality was observed in 48‐h acute toxicity testing when the daphnids were exposed up to 500 µg Ag/L as AgNP. The AgNP accumulation reached as high as 22.9 mg Ag/g dry weight at the highest AgNP concentration tested (500 µg/L). In contrast, D. magna was extremely sensitive to free Ag ion (Ag+, added as AgNO3), with a measured 48‐h 50% lethal concentration of 2.51 µg/L. Thus, any AgNP potential acute toxicity may be caused by the release of Ag+ into the solution. During the 21‐d chronic exposure, dietborne AgNO3 had the most significant influence on reproduction, whereas waterborne AgNP had the most significant inhibition on growth. Significant delay and decrease of reproduction in daphnids exposed to dietborne AgNO3 occurred at a dissolved Ag concentration of 0.1 µg/L added to the algae. Significant inhibitions of growth and reproduction were also found for the AgNP exposure, with the lowest observed effective concentration of 5 µg/L and 50 µg/L, respectively. Chronic effects of AgNP were probably caused by the low food quality of algae associated with AgNP and the low depuration of ingested AgNP. Environmental risk assessments of AgNP should therefore include tests on the chronic toxicity to aquatic organisms as well as the direct and indirect effects of AgNP resulting from the release of Ag+ into the environment. Environ. Toxicol. Chem. 2011; 30:885–892. © 2010 SETAC