[ $alpha$ ]Glucosidase (EC 3.2.1.20) is a glycosidase that hydrolyzes disaccharides, oligosaccharides, and polysaccharides resulting in the release of α-D-glucose. In this study, $alpha$-glucosidase activity in the hemolymph and midgut of the mulberry silkworm Bombyx mori and Japanese oak silkmoth Antheraea yamamai was measured using maltose, sucrose, trehalose, and p-nitrophenyl $alpha$-D-glucopyranoside as substrates. In general, hemolymph $alpha$-glucosidase activity was higher in B. mori than in A. yamamai. In contrast, midgut $alpha$-glucosidase activity was higher in A. yamamai than in B. mori for all of the substrates tested. $alpha$-Glucosidase activity in the midgut of both B. mori and A. yamamai showed similar responses to changes in pH and temperature for all of the substrates tested. Native (7.5%) PAGE of hemolymph and midgut proteins from B. mori and A. yamamai followed by staining with 4-methylumbelliferyl $alpha$-D-glucoside (MUG) indicated that the $alpha$-glucosidases of these related lepidopterans are functionally similar but structurally different. In comparison to $alpha$-glucosidase activity from A. yamamai, $alpha$-glucosidase activity from B. mori was generally less sensitive to the $alpha$-glucosidase inhibitors, 1-deoxynojirimycin (DNJ), acarbose, and voglibose when the activity was determined using maltose, sucrose, and trehalose.