With an increasing use of quantum dots (QDs) in many applications, their potential hazard is of growing concern. However, little is known about their ecotoxicity, especially in vivo. In the present study, we employed freshwater macroinvertebrate, Daphnia magna, to evaluate toxicity characteristics of cadmium selenide/zinc selenide (CdSe/ZnSe) in relation to surface coatings, e.g., mercaptopropionic acid QD (MPAQD), and gum arabic/tri-n-octylphosphine oxide QD (GA/TOPOQD), and light conditions, i.e., dark, fluorescent light, environmental level of ultraviolet (UV) light, and sunlight. The results of the present study showed that D. magna was more susceptible to GA/TOPOQD exposure compared to MPAQD. The surface coating of QD appeared to determine the stability of QDs and hence the toxicity, potentially by size change of or the release of toxic components from QDs. However, GA/TOPOQD was still less toxic than the equivalent level of CdCl2. The toxicity of all the tested compounds increased by changing the light condition from dark to white fluorescence to UV-B light, and to natural sunlight. The effect of light condition on QDs toxicity could also be explained by photostability of the QDs, which would affect size of the particle, release of toxic component ions, and generation of reactive oxygen species. Considering increasing use of QDs in various applications, their environmental fates and corresponding toxic potentials deserve further investigation. © 2009 Wiley Periodicals, Inc. Environ Toxicol 25: 593–600, 2010.