Bacillus thuringiensis (Bt) is an efficient entomopathogen used for pest control. While Bt pathogenesis has been thoroughly studied, less is known about the biological effects of sublethal Bt exposure. To address this knowledge gap, we document sublethal effects of Bacillus thuringiensis var. kurstaki (Btk) on the biology and development of Helicoverpa armigera (Hübner) under laboratory conditions. The results obtained revealed that duration of the different life stages in treated neonates of H. armigera was significantly affected by sublethal treatments. Furthermore, fecundity was also negatively affected in female moths developed from Btk-treated neonates, with the rate of egg hatchability reaching zero in the LC25. We also present data supporting that the effect of sublethal Btk concentrations could carry over to the next generation. Sublethal Btk concentrations reduced the net reproduction rate (R0), and there were also significant differences among the values of this parameter at all treatments tested when compared with controls. The intrinsic and finite rates of increase (rm and λ, respectively) were significantly lower in insects treated with sublethal Btk concentrations compared with control insects. Consequent with the reduce rate of development observed for H. armigera treated with Btk, the mean generation time (T) and doubling time (DT) were significantly higher in insects exposed to any Btk concentration tested compared with control. Our data demonstrate significant fitness costs resulting from sublethal Btk exposure in H. armigera. These observations underline the importance of considering the role of sublethal effects when attempting to evaluate the impact of Bt treatment on an insect pest population and its progeny.