To investigate the effects of Pyriproxyfen (S-31183) on the growth, development and morphogenesis of the desert locust Schislocerca gregaria, five concentration levels (1000, 500, 250, 125 and 62.5 ppm) were given through the fresh clover leaves as food to nymphs of three developmental ages: newly moulted penultimate (4^th) instar, newly moulted last (5^th) instar and late-aged last in star. All results were obtained 24 h after feeding. Pyriproxyfen exhibited a remarkable inhibitory effect on the nymphal growth and consequently nymphs were prevented to obtain more than one-third of weight gain of control congeners at the highest concentration level. Also, the developmental rate was increasingly depressed as the concentration level was increased, almost, in all treatments. The controls had the fastest developmental rate among all nymphs. The adult females were hindered to emerge proprtionally to the increasing concentration level of Pyriproxyfen. Moreover, some adults were completely prevented to emerge after treatment of the penultimate instar nymphs with the highest concentration level. In addition, Pyriproxyfen exerted a profound action on the adult morphogenesis. The degree of adult deformation was dose-dependent after treatment of the newly moulted 4^th or 5^th instar nymphs. After treatment of these nymphs, also, the adult longevity was elaborately shortened while the treatment of late-aged 5th instar nymphs resulted in an insignificant prolongation of the adult longevity.