The histopathological effect of different dosages 50, 100, 200, 500 and 1000 of Steinernema feltiae Filipjev larvae on the larval tissues of Musca domestica Linneous has been studied. It was found that S. feltiae nematodes invade the fat tissue, gut, cuticle and muscle tissue of the host. All of these tissues, along with the gut epithelium, show signs of disintegration before death of the host. The tissue of the gut and the fat body are the most severely damaged by the presence of S. feltae. The destructive damages described in this investigation depend mainly upon the time and the intensity of infection.