The juvenile hormone mimic pyriproxyfen disrupted embryogenesis when applied topically to eggs of the desert locust Schistocerca gregaria Forskal. Eggs treated on days 3-6 were inhibited at various stages of development, depending on dose and age. In particular, 0.001-0.01 mu g blocked development of 3- and 4-d-old eggs at blastokinesis. Treatment of 7- to 11-d-old eggs was ineffective up to 10 mu g. Insects that hatched successfully failed to display any postembryonic defects. Topical application of the mimic to females had a small ovicidal effect. The metamorphic molt was disrupted when the mimic was applied topically to 5th-instar S. gregaria. Insects retained characteristics of the 5th instar and in extreme cases supernumerary 5th instars were formed. Additional defects included essentially normal adults that were malformed and could not fly. Oral doses were considerably less effective. Application to 4th-instar nymphs did not produce supernumerary characteristics in postecdysial insects, though a large proportion of insects showed abnormalities when they reached adult, which in some cases prevented night. Topical application of the mimic to 5th instar affected the length of the instar. The effects depended on dose and day of treatment.