Endo-polygalacturonase from Aspergillus ustus when immobilized on to modified silica gel retained 28% of its original activity. The immobilized enzyme could be re-used through 10 cycles of reaction with almost 90% retention of its original activity. It had increased thermostability over its soluble form: the half-life of the soluble enzyme at 40 degrees C was less than 10 h whereas the immobilized enzyme retained 82% of its activity after 10 h at 40 degrees C. Similarly, at 50 degrees C the half-life of the soluble enzyme was 30 min whereas that of the immobilized enzyme was 5 h.