Thermostable inulinase activity was identified in the extracellular extract of Aspergillus fumigatus. At its optimum temperature of 60 degrees C, the ammonium sulphate fraction retained approximately 70% of its maximum activity after 72 h incubation in the absence of inulin. The two isoforms of A. fumigatus inulinase were purified and their thermostability was studied. In the presence of inulin, isoform II was more thermostable when compared to other two fractions and retained approximately 54% activity after 3h at 60 degrees C. The higher thermostability of inulinase of A. fumigatus makes it a potential candidate for commercial production of fructose.