An extracellular thermostable alpha-galactosidase producing Aspergillus terreus (GR) strain was isolated from soil sample using guar gum as sole source of carbon. It was purified to apparent homogeneity by acetone precipitation, gel filtration followed by DEAE-Sephacel chromatographic step. The purified enzyme showed a single band after sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The molecular weight of the purified enzyme after SDS-PAGE was 108 kDa. The enzyme showed optimum pH and temperature of 5.0 and 65 A degrees C, respectively, for artificial substrate pNP alpha Gal. alpha-Galactosidase from A. terreus (GR) is found to be thermostable, as it was not inactivated after heating at 65 A degrees C for 40 min. The K (m) for pNP alpha Gal, oNP alpha Gal, raffinose, and stachyose are 0.1, 0.28, 0.42, and 0.33 mM, respectively. Inhibitors such as 1,10-phenanthroline, phenylmethylsulfonyl fluoride, ethylenediaminetetraacetic acid, mercaptoethanol, and urea have no effect, whereas N-bromosuccinamide inhibited enzyme activity by 100%. Among metal ions tested, Mg2+, Ni2+, Ca2+, Co2+, and Mn2+ had no effect on enzyme activity, but Ag+, Hg2+, and Cu2+ have inhibited complete activity.