Allosamidin(1), a novel insect chitinase inhibitor, was isolated from the mycelium of Streptomyces sp. and characterized as 1, which was a basic pseudotrisaccharide consisting of 2-acetamido-2-deoxy-D-allose(N-acetyl-D-allosamine) and a novel aminocyclitol derivative(3), termed allosamizoline.Chitin is a main component of insect cuticle. Since insects' growth is accompanied with ecdysis which associates with synthesis and degradation of chitin1), the inhibitors against chitinase, which hydrolyze chitin into its monomer, 2-acetamido-2-deoxy-D-glucose(N-acetyl-D-glucosamine), should be the models for designing a new type of insect growth regulators. From this point of view we have been continuing to search the chitinase inhibitors among microbial products and found that the mycelial extract of Streptomyces sp. markedly inhibited the chitinases of the silkworm, Bombyx mori, in vitro and prevented its larval ecdysis in vivo2). In this paper we wish to report preliminarily the structural elucidation of the active principle, termed allosamidin(1).