The toxicological effect of two pesticides, pyriproxyfen and abamectin were eva-luated on third instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.). According to the estimated toxicity values, i.e. LC50, LC90 and slope values, the chemical abamectin showed that it was slightly more toxic than pyriproxyfen. The effect of these two insecticides, at the determined LC50 and LC25 values on the di-gestive physiology of treated larvae, as indicated by some nutritional indices was conducted. Obtained results demonstrated that pyriproxyfen and abamectin caused a significance reduction in growth of treated larvae as depicted by larval weight gain Also, a significant decrease was found in the efficiency of ingested food to body matter and efficiency of conversion of digested food to body matter as compared to untreated insects. These effects were generally more evident when abemectin was used. The two tested compound also caused histological changes in the midgut of treated larvae, in form of disruption in the columnar epithelium cells and stretching leading to tearing in the peritrophic membrane. This observation might explain the impairment in nutritional indices in treated larvae as compared to the control.