Farinocystis tribolii multiplied only in the fatbody of Tribolium castaneum larvae. In the advanced stages of infection, the fatbody was destroyed and the albuminoid granules were completely depleted. Larvae in late stages of infection were “C” shaped. Diseased pupae were flat and straight with the head and mouth parts not completely flexed with the pupal body. Infected pupae failed to develop into adults. Heavily infected adults were distended. Larvae of third, fourth, and fifth instars, although succumbing to the infection ultimately, continued to live even after the healthy larvae of the same age had entered pupation, indicating a prolongation of larval period. Molting was either affected or delayed in infected insects. F. tribolii infection induced a juvenile hormonal effect producing larval-pupal and pupal-adult intermediate forms. Ether extract of spores of F. tribolii when applied on pupae of T. castaneum and final instar nymphs of Dysdercus cingulatus produced adultoids.