An ${alpha}$-glucosidase inhibitor, SKG-3, was isolated from the fruiting bodies of Ganoderma lucidum and its physico-chemical properties were characterized. It was a highly specific and effective reversible inhibitor of ${alpha}$-glucosidase. It showed very potent inhibitory activity against a-glucosidase with an IC$sub$50/ value of 4.6$mu extrm{g}$/$mell$, but no activity for any other glycosidases tested. Enzyme activity could be recovered upon dialysis, thus providing evidence for the reversibility of the inhibition. A Lineweaver-Burk plot indicated that the SKG-3 inhibition of ${alpha}$-glucosidase was competitive.