N-Acetylgalactosamine-based 1-N-iminosugars, new types of glycosidase inhibitor have been synthesized by modeling on siastatin B, isolated from a Streptomyces culture. The analogues of siastatin B were proved to be potent inhibitors for alpha-N-acetylgalactosaminidase and/or beta-N-acetylglucosaminidase.