The toxicity of silver nanoparticles (AgNPs) has been widely investigated, but the process of bioaccumulation such as uptake remains less studied. In the present study, we employed the radioisotope ((110m)Ag) to trace the behaviors of commercial AgNPs with three nominal particle sizes (20, 50, and 100 nm) and two surface coatings (citrate and tannic acid) in a model organism Daphnia magna . The size distributions of AgNPs in the medium increased continuously as the exposure time increased, especially for the smallest AgNPs (20 nm). Cysteine, the amino acid containing thiol group, significantly enhanced particle aggregation, with a 30-fold increase of the hydrodynamic size for AgNPs with 20 nm nominal size after 6 h of exposure. We demonstrated that the influx rates of AgNPs into daphnids were size-dependent. At 500 μg/L AgNPs with 1 μM cysteine, the influx rates of AgNPs were in the sequence 20 nm > 50 nm > 100 nm (nominal size) for both types of surface coatings. Such sequence was consistent with the size distribution in the medium. More than 60% of AgNPs were distributed in the gut of daphnids, indicating that ingestion was the dominant uptake pathway. The size-dependent influx rate was also observed at different AgNPs concentrations. The measured uptake rate constant was lower than that of AgNO(3) at low AgNPs concentration, but it became higher at high AgNPs concentrations. Our study highlighted the indispensability of characterizing the size distribution of AgNPs dispersed in the medium in studying the AgNPs uptake. The accurate quantification of AgNPs influx rate suggested an uptake pathway entirely different from that of AgNO(3) in the daphnids.