We showed that an oral administration of a fungicide containing iminoctadine triacetate had a delayed effect on the development of Bombyx mori (L.) larvae. Larvae that ingested a diet with a moderate dose of formulated-iminoctadine triacetate throughout the third instar stage developed to the next (fourth) instar stage; thereafter they stopped growing. Those larvae died finally after prolonged fourth instar duration without molting to the next (fifth) instar stage. Dietary intake of the larvae showing the growth inhibition was significantly lower than that of the control group. Fourth instar larvae were reared with restricted amount of diet that was comparable to that of formulated-iminoctadine-treated specimens. These dietary restricted larvae grew at a slower pace and reached a lower maximum weight than controls; however, they developed to the next (fifth) instar stage. Nevertheless, the weight of which was smaller than the control group. These results suggested that reduced food intake did not cause the failure to molt to fifth instar. In the larvae affected by formulated-iminoctadine, “housekeeping genes” (18S ribosomal RNA, actin 3, elongation factor 2, and nsd-2) were expressed normally. From these results, it is assumed that the formulated-iminoctadine-treated larvae lost the capacity of high assimilatory performance and could not build up their body mass furthermore.