A systematic screening of the toxicity of ionic liquids (IL) towards Vibrio fischeri, a bioluminescent marine bacteria generally used in ecotoxicological bioassays, was carried out. The objectives of this work were to find hydrophilic or hydrophobic low toxicity IL and to investigate structure-toxicity relationship of IL Toxicity of 54 IL to V. fischeri have been measured, some referring to new IL based on quinuclidinol or tropinol and some to generic IL (i.e., imidazolium, pyridinium, pyrrolidinium or piperidinium). For 47 of them, toxicity values have not been reported elsewhere. Water-soluble IL containing hydrophilic anions halide, thiocyanate, dicyanamide, trifluoromethansulfonate were studied. Some IL were found to exhibit very low toxicity towards V. fisheri. Hydrophobic IL based on bis(trifluoromethanesulfonyl)imide, tetrafluoroborate tetraphenylborate and tetracyanoborate were also studied. Toxicity was measured in a consistent way starting from aqueous solutions saturated with IL. The least toxic hydrophobic IL found in this study was [EMIM][B(CN)(4)]. A multifactorial analysis was found to be convenient for finding relevant structure parameters influencing the toxicity of IL. From this analysis, the planarity of the cation ring appeared to be a relevant parameter. Finally, good linear correlations were found when toxicity of IL was plotted either against the number of aliphatic carbons surrounding a pyridinium cation or the total number of carbons of a cation. (C) 2012 Elsevier B.V. All rights reserved.