Effect of exogeneus juvenile hormones (JHs) on pupal development was assayed in the beet armyworm, Spodoptera exigua. Fenoxycarb, a potent JH mimic, was applied topically to different ages of the pupae, and showed significant inhibition of normal adult eclosion even at 0.1$mu extrm{g}$ dose when it was applied at the early pupal stage (day 0). As the pupal development underwent, the susceptibility of the pupae fenoxycarb decreased. RH5992, a potent ecdysteroid mimic, did not, however, any similar inhibiotry effect on the pupae. Natural JH types (JH I, JH II, and JH III) were applied on day 0 pupae to compare their inhibitory effects en adult eclosion. Both JH I and JH III significantly inhibited adult eclosion at 1.0$mu extrm{g}$ dose, but JH III did not even at 10.0$mu extrm{g}$ dose. It was meted that fenoxycarb-treated pupae showed little rectum development. Fenoxycarb did not, however, show any negative effect on the development of compound eye and wing imaginal discs, and on the pupal hemolymph protein pattern. These results suggest that there should be a commitment period requiring an absence of JH fur a normal adult metamorphosis during early pupal development and that the endogenous type of JH in S. exigua is JH I or JH II or both JHs like other lepidopteran species.