In the present study, the infectivity of the three entomopathegnic nematodes, Steinernema feltiae, Steinernema riobrave and Heterorhabditis bacteriophora, on the cotton leafworm, Spodoptera littoralis was studied. Moreover, the effect of these pathogens on certain biochemical and physiological aspects of the host was also studied. H. bacteriophora, appeared to be more pathogenic than S. riobrave and S. feltiae to the S. littoralis larvae. The highest production (7000 infective juveniles) was obtained, where infective juvenile production from cadavers infected with H.  bacteriophora was higher than that produced from cadavers infected with S. riobrave and S. feltiae. The principle nutrients (total protein, carbohydrate and lipid) of the host larvae were highly decreased post-infection with the nematodes S. riobrave and H. bacteriophora. The activity of some larval enzymes was also affected due to infection by these nematodes. Thus, the activity of carbohydrate hydrolyzing enzymes (amylase, invertase and trehalose) changed depending on the species of the pathogen and the enzyme. Amylase activity decreased with the infection by H. bacteriophora, and the reverse was obtained with the infection by S. riobrave and S. feltiae where such activity increased. Invertase and Trehalase activity increased with the infection by three nematode species. The highest increase was obtained in case of infection by S. riobrave. Activities of acid and alkaline phosphatases increased due to infection by S. riobrave, H. bacteriophora. The only exception was a non-significant decrease in the alkaline phosphatase activities of larvae infected with S. feltiae. Whereas, the activity of transaminases (GOT and GPT) was highly decreased with the infection by H. bacteriophora and S. riobrave.