In insects, juvenile hormone (JH) facilitates postembryonic development but prevents metamorphosis. In the embryos of hemimetabolous insects, however, early JH exposure arrests embryonic growth and tissue patterning and elicits premature differentiation [Truman and Riddiford, Phil. Trans. R. Soc. B 374: 20190070 (2019)]. We show here that in the basal, ametabolous firebrat, Thermobia domestica, application of JH III or the JH mimic (JHM) pyriproxyfen early in embryogenesis allows normal growth through the extended germband stage and secretion of the first embryonic cuticle; but then embryo differentiation stalls and limbs regress. Developmental anomalies are reduced as JHM treatment is delayed, and sensitivity to exogenous JHM is lost at dorsal closure as a second embryonic cuticle (cuticle of the first postembryonic instar) starts to be secreted and endogenous JH appears. Thus, as in embryos of basal hemimetabolous insects, JH can suppress early morphogenesis; but it has less effect on precocious differentiation as compared to locusts or crickets. JHM treatment beginning at hatching has no effect on the first two non-feeding instars, but the 3rd instar has an altered morphology, including reduced size, incomplete sclerotization of the mandibles and, often, reduced or no feeding. To elucidate the underlying molecular basis of the embryonic and postembryonic effects, we studied the mRNA profiles of the nymphal transcription factor (TF) Broad and the Krüppel homolog 1 (Kr-h1) TF induced by JH. Broad is maternally deposited in the egg, then declines during early development only to reappear as the germ band develops, then persist through most of nymphal development. It is also present during the nymphal intermolts. The regulation of these two TFs by the exogenous JH will be discussed.