It is a well known fact that some ionic liquids (ILs) have the potential to dissolve crystalline, water-insoluble cellulose that could be used as a source of glucose and subsequently derived molecules. Nevertheless, in the presence of high IL concentrations, cellulase activity and, even more so, cellulase stability are still challenging difficulties that need to be overcome. Therefore four fungal cellulase preparations were assayed in the presence of 30% (v/v) of five different ILs. Thereby the cellulase from Aspergillus sp. (Sigma-Aldrich) was not only remarkably active (up to 28 U/mg in 30% (v/v) IL ([BMMIM]Cl)), but furthermore stayed active during several weeks in the presence of 60% (v/v) IL (up to 50% relative activity after 9 weeks).