gem-Diamine 1-N iminosugars of D-glucose-type, a new type of glycosidase inhibitors, have been synthesized from siastatin B, isolated from Streptomyces culture. 2-Trifluoroacetamido-1-N-iminosugar, (2S,3R,4R,5R)-2-trifluoroacetamido-5-hydroxymethylpiperidine -3,4-diol was proved to be a potent inhibitor for alpha-D- and beta-D-glucosidases (IC50 1.9x10(-7) and 4.2x10(-7) M, respectively). 2-Acetamido-1-N-iminosugar, (2S,3R,4R,5R)-2-acetamido-5-hydroxymethylpiperidine-3,4-diol also affected these enzymes (IC50 2.9x10(-6) and 5.4x10(-6) M, respectively).