The lepidopteran stemborers Busseola fusca (Fuller) (Lepidoptera: Noctuidae), Sesamia calamistis (Hampson) (Lepidoptera: Noctuidae), and Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) are effectively controlled by Bt maize that expresses the Cry1Ab insecticidal protein. Another noctuid species, the cutworm Agrotis segetum (Denis and Schiffermüller) (Lepidoptera: Noctuidae), which is the most common and injurious pest of maize seedlings in South Africa, is exposed to Bt toxin for a part of its life cycle. The effect of this exposure to Bt maize has not been studied yet. The aims of this study were to determine the effects of Bt maize (events MON810 and Bt11) on larval mass, development time, survival, and fecundity of A. segetum. Laboratory studies were conducted with first- and fourth-instar larvae and moths. Results showed that the effect of Cry1Ab toxin on the biology of A. segetum larvae and moths were largely insignificant. The effects of the two Bt maize events on the different parameters measured in this study was not similar between the Bt events and their respective iso-hybrids. Compared with larvae that fed on conventional (non-Bt) maize, Bt maize did not affect survival of first-instar larvae. However, mean mass of larvae that fed on Bt maize (Bt11) was significantly lower. Feeding on Bt maize did not have a significant effect on development and survival of fourth-instar larvae or moth longevity. It did, however, delay the development period to pupa formation. Fewer eggs were laid by moths fed as larvae on maize event Bt11 compared with MON810. This study indicates that Bt maize will most likely not have any significant effect on the control of A. segetum under field conditions.