In the face of increasing applications for nanotechnology, the potential for nanoparticles to bioaccumulate continues to be a major knowledge gap. The aim of this project was to evaluate bioconcentration and biomagnification of nano-cerium dioxide (CeO2, uncoated, mean particle size 28 nm) in fish via water and diet respectively. Carp (Cyprinus carpio) were exposed to treatments of nano-CeO2 (1 & 100 mg/L) and micro-CeO2 (100 mg/L, mean particle size 200 nm) dispersed in water containing 10 mg/L humic acid for 10 days. As a separate treatment, carp were fed nano-CeO2 at 68 mg/kg in diet for 10 days. Fish samples were collected on 3 occasions (at day 1, 4, 10), and water samples every third day. All samples were immediately analyzed for Ce by ICP-MS. The uptake of Ce was evaluated in selected tissues (liver, gastrointestinal tract, gills, skin and filet) and the carcass from dissected fish. The highest amounts of Ce were found in the gastrointestinal tract in both feeding and water exposures. In skin and gills considerably lower amounts of Ce were found. In internal tissues such as liver and filet measured Ce was generally below the quantification limit, with a few exceptions that were most likely due to contamination during dissection. The results indicate that there is a very low bioaccumulation potential for CeO2 (BCF <5 and BMF <0.002). There was no difference in accumulation between nano- and micro-CeO2.