The histopathological effect of hematoporphyrin on the mid-gut and the integument of the fourth larval instar of C. pipiens was studied using a transmission electron microscope. In this study C. pipiens larvae were treated with different concentration of hematoporphyrin. The concentration 5 × 10-4 M/L was found to produce 100% mortality after half-hour exposure to the 400 W/M2 artificial lights using a solar simulator. The ultrastructural examination showed that the normal lamellate cuticle was heavily affected, taking the shape of amorphous cuticular region. The endo- and exo- cuticle were not distinguishable. It was found that the epidermal cells underneath the cuticle were distorted. The mitochondria of these cells exhibited irregular shapes. The fat body underneath the integument could be detected with visible vacuolisation. Muscle cells revealed the degenerated sarcomeres with gaps and vacuoles. The Z discs have irregular shape and are distorted. Mid-gut cells appeared with cytoplasmic vacuoles. The Golgi body is fragmented into small bodies. The rough endoplasmic reticulum is broken-down into separate vascular structures. These cytopathological observations confirm the insecticidal efficiency of hematoporphyrin against C. pipiens larvae.