This study was conducted to determine the effects of diflubenzuron (DFB) on the growth and development of the silkworm, Bombyx mori (L.). The DFB treatment extended abnormally the larval duration and affected negatively on larval spinning of the 5th instar. All the larvae treated with high DFB concentration (2.5$ imes$10$^{-1}$ and 2.5 $ imes$10$^{-1}$ $mu extrm{g}$/${mu}ell$) lost their spinning capability and finally died, whereas 62% of larvae treated with low DFB concentration (2.5$ imes$10$^{-1}$ $mu extrm{g}$/${mu}ell$) spinned cocoon. The larval weights depended sensitively on the DFB treatment period rather than on the DFB concentration. The DFB treatment decreased the larval maturity less than 6% without regard to the concentration and treatment period. All the larva, when treated with DFB before the 5th days of the 5th instar, were not matured.