In this study, the effect of ZnO nanoparticles and ZnCl2 on growth, reproduction and accumulation of Daphnia magna was determined in a 21 day chronic toxicity test. Different techniques were used to distinguish between the Zn2+, dissolved, nanoparticle and aggregated fraction in the daphnia test medium. The results showed similar chronic effects on growth, reproduction and accumulation for the ZnO nanoparticles (EC10, 20, 50 reproduction: 0.029, 0.048, 0.113 mg Zn/l) and the ZnCl2 (EC10, 20, 50 reproduction: 0.014, 0.027, 0.082 mg Zn/l). A large fraction of the nanoparticles rapidly dissolved after introduction in the exposure medium. However, also aggregation of nanoparticles was observed but after 48 hours of exposure most of these ZnO aggregates were dissolved. Based on the combined dissolution kinetics and toxicity results it can be concluded that the toxicological effects of ZnO nanoparticles at the chronic level can be largely attributed to the dissolved fraction rather than the initially formed aggregates.