In this study, a specifically-designed aggregation-induced emission fluorogen (AIEgen) with nanoparticle aggregates was used to quantitatively evaluate the bioaccumulation of Hg 2+ and visualize Hg 2+ kinetics in vivo within the rotifer Brachionus plicatilis for the first time. Quantitative results showed that a sharp drop in Hg 2+ concentration occurred at the very beginning in the medium containing rotifers and Hg 2+ , showing a quick initial uptake of Hg 2+ by the rotifers, and then the concentration in the medium plateaued after 5 min. With an increase in rotifer density, the amount of bioaccumulation increased in the rotifer. However, the bioaccumulation efficiency of Hg 2+ decreased from 5.28 μg mg -1 h -1 at a low rotifer density of 0.093 mg ml -1 to 2.61 μg mg -1 h -1 at a high rotifer density of 0.375 mg mL -1 . Moreover, the fluorescence images and spectra results illustrate that the ingestion of Hg 2+ by the rotifer was via its mouth surrounded by the ciliary corona to the digestive tract, and Hg 2+ could not permeate into the body integument through diffusion during the study period. Hg 2+ -induced fluorescence in rotifers dissipated in 6 h after staining, possibly through defecation and excretion. This study indicates that inorganic mercury can be quickly ingested by a rotifer via feeding, but is unlikely deposited as methylated mercury in rotifer tissues.