Experiments were conducted to assess the impact of Bacillus thuringiensis isolate against the midgut of larvae for pink bollworm (PBW) Pectinophora gossypiella (Saund.). The data illustrated indicated that the LC₅₀ value of the testedpotent isolate was 3.77 CFU/ml; the confidence limits at (95%) were 2.93- 4.91CFU/ml. In addition, LC₉₀ value also determined and it obtained 44.23 CFU/ml. and its confidence limits at (95%) were 23.18- 149.70 CFU/ml. The effects of the bacterial isolate on the midgut of 4^th larval instar of the pink bollworm,treated with LC₅₀CFU/ml showed several histological changes some epithelial cells were disintegrated, vacuolated and their cell boundaries were destructed and separated from the basement membrane. The peritrophic membrane was also destructed and detached from the epithelial cells. In general, epithelium layer in untreated specimen was thicker than that in bacterial treated one. The mode and site of action of the active isolates on PBW larvae have been investigated with Transmission Electron Microscope. The Columnar epithelial cells of the midgut showed the muscle layer surrounding the basement membrane lost its normal appearance and degenerated. An increase of deeply infoldings of the basement membrane occurred and the mitochondria of the basal regions were transformed into a condensed form. Separation in columnar and goplet cells occurred and cleared scatting into the gut luman. Microvilli were disrupted showing non-altered appearance. Strong alterations of cytoplasmic structure and organelles occurred. Also, the cytoplasm was distinguished by numerous vacuoles and destroyed brush border. Fragmented chromatin inside the nucleus that has a terminal position destroyed the nucleus sheath.