The acid and alkaline phosphatase activities were studied in the developing egg of the cotton leaf worm, Spodoptera littoralis at different time intervals (ages, 0 - 72 h). The activities of these enzymes were also studied in aging ovary and testis of post-emerged adult at the same intervals (ages, 0 – 72 h). The post-emerged adults were grouped into mated and non-mated females and males. Acid phosphatase activity was noticed to be cyclic with the embryonic development and was higher than that of alkaline phosphatase. Alkaline phosphatase activity was slightly increased in the freshly laid eggs but decreased more sharply than acid phosphatase during embryonic development. In all groups of post-emerged adults, acid phosphatase activities were much greater than those of the alkaline phosphatase at any given stage of progressively increasing age. Acid and alkaline phosphatase activities were also noticed to be cyclic in the post-emerged mated and non-mated adults. A conspicuous difference was noted between the activities of these enzymes in ovaries and testes of mated and non-mated adults. This phenomenon could be due to the periods of spermatozoa production in the testes and yolk accumulation in oocytes and ovulation. Mating could be also a factor that affected the phosphatases in both males and females.