Ionic liquids (ILs) can be considered as environmentally friendly solvent, but they have the ability to dissolve in water and
accumulate in the environment. Therefore, the toxicity of ILs should be assessed in order to prevent their harm to human and
environment. This study was carried out to investigate the toxicity of ILs towards marine and freshwater fish. Three ILs have
been tested, which are 1-Butyl-3-methylimidazolium hydrogen sulfate and 1-Butyl-3-methylimidazolium bis
(trifluoromethylsulfonyl) imide toward marine fish and 1-Hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide toward
freshwater fish. Two different marine fish were employed, which are: Cephalopholis cruentata (grouper) and Lates calcarifer
(barramundi). For freshwater fish, male Poecilia reticulate (guppy) was employed. The toxicity tests were conducted according
to OECD (Organisation for Economic Cooperation and Development) guideline 203. For 1-Butyl-3-methylimidazolium hydrogen
sulphate [BMIM][HSO4], the median lethal concentration (LC50) estimated toward Cephalopholis cruentatato be 199.98 mg.L-1.
For 1-Butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [BMIM][TFSI], LC50 estimated toward LatesCalcariferto be
374.11 mg.L-1. While, for 1-Hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, [HMIM][NTf2], LC50 estimated
toward Poecilia Reticulate to be 207.49 mg.L-1. All the LC50 values obtained can be identified as practically nontoxic liquids
based on Acute Toxicity Rating Scale by Fish and Wildlife Service (FWS). As to our knowledge, there is no previous reported
toxicity studies of [BMIM][HSO4] and [BMIM][TFSI] on marine fish and [HMIM] [NTf2] on freshwater fish. Thus, this paper
can be used as a benchmark for researchers who are dealing with these three ILs.