Consequent to injection of conidia at LC30, LC50 and LC90 doses of the entomopathogenic fungal isolates of Metarhizium anisopliae (Metch.), alterations in lipid peroxidation and ultrastructural changes were observed in cockroaches (Periplaneta americana). A decrease in the level of lipid peroxidation was evident in the treated cockroaches compared to those of control. Present investigation reports the effects of fungal infection on the midgut ultrastructure of the adult cockroaches which was manifested in the form of damage to the microvilli, epithelial cell vacuolization, necrosis and disruption of the epithelial cell membrane which occurred with increase in the time interval post treatment with conidia from high virulent isolate (M20) at LC50 dosage. Our study reveals for the first time that fungal infection instigates oxidative stress in the cockroach and that the villi of the midgut are also the target organs for the oxidative damage.