In situ extractive fermentation of lactic acid using organic solvents has already been heavily investigated. Now ionic liquids are emerging as alternative solvents for volatile organic compounds traditionally used in liquid-liquid extraction. In this paper, we examine whether imidazolium-based ionic liquids can replace conventional organic solvents in the extractive fermentation of lactate by investigating their extraction behaviors and solvent toxicity. The extractions of organic acid using imidazolium-based ionic liquids without any extractant are controlled by the hydrophobicity of the acids, and in all cases the extractability is very low. The extraction behaviors of lactic acid with imidazolium-based ionic liquids containing tri-n-butylphosphate extractant are similar to those of conventional organic solvents. Lactic acid producing bacterium, Lactobacillus rhamnosus NBRC 3863, grew, consumed glucose and produced lactate in the presence of imidazolium-based ionic liquids. These findings suggest that ionic liquids could be used in an in situ extractive fermentation process. (C) 2004 Elsevier B.V. All rights reserved.