The present study was devoted to elucidate the mode of action and histopathological effects of the spore δ-endotoxin complex of B.t. var aizawai on the larvae of the cotton leaf worm Spodoptera littoralis.The results obtained has clearly shown that the consequence of symptoms of infection could be divided in morphologically distinct four stages. Furthermore, the results of this investigation have indicated that the sequence of symptoms starts with cessation of feeding followed by movement sluggishness, vomiting and diarrheae, excessive sluggishness, complete paralysis and finally death of the insect. On the other hand, the histopathological effects of the endotoxin observed in the dissected insect fed on diet containing the toxin were followed periodically. The results have clearly demonstrated marked histopathological alterations in the midgut epithelium, layers and clumping of both exo-and endocuticle of the integument. Furthermore, the uptake of bacterial δendotoxin has caused a marked degeneration of the nerve cells of the fourth abdominal nerve ganglion. The nerve cells and fibers were partially destroyed. In addition a notable destruction and vacuolation of the fat body cells became evident and the fat tissues became soft and easily crushed as compared to those of the healthy insect.