A screening method for alpha- and beta-D-glucosidase inhibitors was developed by using agar plates. An enzyme agar plate containing either alpha- or beta-D-glucosidase was used in combination with a substrate agar plate containing p-nitrophenyl alpha- or beta-D-glucopyranoside. Paper disks impregnated with algal extracts were placed on the enzyme agar plate, then preincubated for 2.5 h at 25-degrees-C. The substrate agar was layered on the enzyme agar. Inhibitory zones appeared colorless in the yellow background of p-nitrophenol. Extracts of algae collected in southern Hokkaido were screened for alpha- and beta-D-glucosidase inhibitors by the proposed agar plate method. Among nine algal species examined, seven species inhibited alpha-D-glucosidase, where five species inhibited beta-D-glucosidase.