The incidental ingestion of engineered nanoparticles (NPs) can be an important route of uptake for aquatic organisms. Yet, knowledge of dietary bioavailability and toxicity of NPs is scarce. Here we used isotopically modified copper oxide (⁶⁵CuO) NPs to characterize the processes governing their bioaccumulation in a freshwater snail after waterborne and dietborne exposures. Lymnaea stagnalis efficiently accumulated ⁶⁵Cu after aqueous and dietary exposures to ⁶⁵CuO NPs. Cu assimilation efficiency and feeding rates averaged 83% and 0.61 g g^–1 d^–1 at low exposure concentrations (<100 nmol g^–1), and declined by nearly 50% above this concentration. We estimated that 80–90% of the bioaccumulated ⁶⁵Cu concentration in L. stagnalis originated from the ⁶⁵CuO NPs, suggesting that dissolution had a negligible influence on Cu uptake from the NPs under our experimental conditions. The physiological loss of ⁶⁵Cu incorporated into tissues after exposures to ⁶⁵CuO NPs was rapid over the first days of depuration and not detectable thereafter. As a result, large Cu body concentrations are expected in L. stagnalis after exposure to CuO NPs. To the degree that there is a link between bioaccumulation and toxicity, dietborne exposures to CuO NPs are likely to elicit adverse effects more readily than waterborne exposures.