The structures of allosamidin (1) and methylallosamidin (2), novel insect chitinase inhibitors, were elucidated as 1 and 2 by acid hydrolysis experiments and analyses of 2D-NMR spectra. They are unique basic pseudotrisaccharides consisting of 2-acetamido-2-deoxy-D-allose (N-acetyl-D-allosamine) and a novel aminocyclitol derivative (3), termed allosamizoline.