It is inevitable that carbon nanotubes (CNTs) will be released to and widely dispersed in environmental ecosystems, given their numerous expected applications. Determination of their potential for bioaccumulation by ecological receptors is thus critical. Previous, research involving several different terrestrial and benthic organisms has indicated that CNTs spiked to soils or sediments do not bioaccumulate. Conversely, we report here distinctly different uptake and depuration behaviors for an aquatic organism, Daphnia magna, in a water-only system. After 48 h of exposure of this organism to a 0.4 microg/mL solution of dispersed nanotubes, the CNTs comprised 6.3 +/- 1.5% of the residual organism dry mass. Moreover, these organisms were unable to excrete the nanotubes to either clean artificial freshwater or filtered Lake Kontiolampi water after 24 h depuration periods, even though the lake water had a substantial concentration of natural organic matter. Addition of algae to the water during the depuration period did result however in release of a significant fraction (approximately 50-85%) of the accumulated CNTs within the first few hours, but little thereafter. Light microscopy results suggest that the vast majority of the accumulated CNTs remained in the organisms' guts and were not absorbed into cellular tissues.