We report that several tryptophan-rich peptides exhibit an affinity for a hydrophobic ionic liquid (IL) (1-ethyl-3-methylimidazolium bis-trifluoromethanesulfonyl imide), and that green fluorescent protein (GFP) fused to a peptides, "SSSWWSWWWW" (SW1) or "SWWWWSWWWW" (SW2), containing serine (S) and tryptophan (W) at the C terminus localized at the IL/water interface. While GFPs without W-rich peptide distributed only in water phase, SW1- and SW2-GFPs were accumulated at the interface. The localization of SW1-GFP showed biphasic behavior, and most distinctive localization was observed at 7.1 mu M. The localization of SW2-GFP presumably occurred at largely lower concentration (<= 0.5 mu M) than that of SW1-GFP, which difference was due to the higher hydrophobicity of SW2 peptide. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.