Water-soluble fullerene (nC60) has been shown to induce lipid peroxidation (LPO) in brain of juvenile largemouth bass (LMB, Micropterus salmoides) [Oberdörster, E., 2004. Manufactured nanomaterials (fullerenes, c60) induce oxidative stress in brain of juvenile largemouth bass. Environ. Health Persp. 112, 1058-1062]; and upregulate genes related to the inflammatory response and metabolism, most notably CYP2K4 [. Nanotoxicology: an emerging discipline evolving from 116 studies of ultrafine particles. Environ. Health Persp. 113, 823-839]. The initial study in LMB was performed using tetrahydrofuran (THF)-solubilized nC60, although C60 can also be solubilized by stirring in water. The current study investigates differences in acute toxicity to Daphnia magna between THF-solubilized and water-stirred-nC60 as a range-find for further assays in adult male fathead minnow (FHM, Pimephales promelas). The daphnia 48-h LC50 for THF-nC60 was at least one order of magnitude less (0.8 ppm) than that for water-stirred-nC60 (> 35 ppm). FHM were dosed with either 0.5 ppm of THF- or water-stirred-nC60 for 48 h. There was 100% mortality in the THF-nC60-exposed fish between 6 and 18 h, while the water-stirred-nC60-exposed fish showed no obvious physical effects after 48 h. Water-stirred-nC60 elevated LPO in brain, significantly increased LPO in gill, and significantly increased expression of CYP2 family isozymes in liver as compared to control fish.