The insecticidal endotoxin crystals of Bacillus thuringiensis var israelensis (HL-75) which was newly isolated were fed to 3rd instar lavae of Culex pipiens mosquitoes and then the midgut cells of the mosquitoes were visualized to observe histopathological changes with a transmission electron microscope. B. thuringiensis endotoxin crystals induced a cyto-plasmic disintegration of the tissue cells in the midgut of the mosquito. Microvilli became deformed in size and configuration and their organized internal microfilaments were disrupted. Some of the microvilli were destroyed in part or entirely.