Room-temperature ionic liquids (ILs) are considered to be green chemicals that may replace volatile organic solvents currently used by industry. However, IL effects on aquatic organisms and ecosystems are currently unknown. We studied the acute effects of imidazolium-based ILs on survival of the crustacean Daphnia magna and their chronic effects on number of first-brood neonates, total number of neonates, and average brood size. Lethal concentrations of imidazolium ILs with various anions (X-) ranged from a median lethal concentration (LC50) of 8.03 to 19.91 mg L(-1), whereas salts with a sodium cation (Na+ X-) were more than an order of magnitude higher (NaPF6 LC50, 9,344.81 mg L(-1); NaBF4 LC50, 4765.75 mg L(-1)). Thus, toxicity appeared to be related to the imidazolium cation and not to the various anions (e.g., Cl-, Br-, PF6-, and BF4-). The toxicity of imidazolium-based ILs is comparable to that of chemicals currently used in manufacturing and disinfection processes (e.g., ammonia and phenol), indicating that these green chemicals may be more harmful to aquatic organisms than current volatile organic solvents. We conducted 21-d chronic bioassays of individual D. magna exposed to nonlethal IL concentrations at constant food-resource levels. Daphnia magna produced significantly fewer total neonates, first-brood neonates, and average neonates when exposed to lower concentrations (0.3 mg L(-1)) of imidazolium-based ILs than in the presence of Na-based salts at higher concentrations (400 mg L(-1)). Such reductions in the reproductive output of Daphnia populations could cascade through natural freshwater ecosystems. The present study provides baseline information needed to assess the potential hazard that some ILs may pose should they be released into freshwater ecosystems.