Toxicological, biological and histological effects of the isolated entomopathogenic fungus Metarhizium anisopliae from the red palm weevil adult on the newly molted 2nd and 4th instar larvae of the Egyptian cotton leaf worm, Spodoptera littoralis (Boisd.), were investigated under laboratory conditions at 27º ± 2º and 60% ± 5% R.H. Results revealed a decrease in the mean larval, pupal, and adult durations for the 2nd and 4th instar larvae surviving treatment with the LC50 of 2.8x108 and 2.8x109 spore/ml for the two tested instar larvae, respectively. Treatment also caused a reduction in the pupation and adult emergence percentage. There was also a great reduction in the mean number of eggs/female and the mean number of hatched eggs; The histological changes in the mid gut tissues in 6th instar surviving treatment with LC50 of M. anisopliae as newly ecdysed 3rd instar larvae was investigated. Sections in mid gut of untreated S. littoralis showed linning with columnar cells surrounded by cross longitudinal muscles fiber, the epithelial cells linning mid gut with its brush border and goblet cells. While, in mid gut of treated S. littoralis some epithelial cells which linning mid gut have lost their brush border, others were completely destructed.