The black cutworm Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is generally considered to be worldwide in distribution. It is one of the destructive pests attacking nearly all vegetables and different field crops. The objective of the present study was to evaluate the toxicity of pyriproxyfen and its effect on the growth, development, metamorphosis and morphogenesis of this insect. Both the 4^th instar and 5^th instar larvae were treated with 6 concentrations of this juvenoid (800, 400, 200, 100, 50 & 25 ppm) via fresh discs of castor bean leaves. The most important results could be summarized as follows. Pyriproxyfen exhibited strong acute toxic activity against larvae and chronic toxicity against pupae and adults, after treatment of 4^th instar or 5^th instar larvae. LC₅₀ values were calculated in 65.95 and 99.90ppm, after treatment of 4^th instar and 5^th instar larvae, respectively, i.e., the 4^th instar larvae were found more sensitive to pyriproxyfen than 5^th instar larvae. The larval body weight gain was remarkably reduced and the growth was considerably inhibited. The larval and pupal durations were considerably prolonged, in a dose-dependent course. Failure of ecdysis, as a criterion of the disrupted developmental program, was observed only after treatment with certain concentrations, but other features of the disrupted program had not been observed. The pupation was detrimentally suppressed after treatment of 4^th or 5^th instar larvae with pyriproxyfen. No deformed pupae were observed. Therefore, pyriproxyfen could be recommended as an eco-friendly alternative to synthetic insecticides for the IPM program of A. ipsilon.