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This study was done to compare the mortality of Helicoverpa zea (Boddie) eggs and larvae of different ages on cotton treated with Bacillus thuringiensis Berliner var. kurstaki. Mortality of H. zea neonates hatching from eggs collected from B. thuringiensis-treated cotton plots was significantly (P ≤ 0.01) higher for eggs 0 than 2 d in age at the highest (1.12kg/ha) rate, but did not differ at lower rates. However, across all rates the median lethal concentration (LC50) was 3.2-fold lower for larvae hatching from eggs that were 2 d in age than those from eggs 0 d in age. Mortality for larvae 1 d in age placed on B. thuringiensis-treated cotton terminals was significantly higher than for larvae 3 and 5 d in age at 4.48 and 8.96 kg/ha rates of B. thuringiensis. The LC50 for larvae 1 d in age (2.1 kg/ha) was 3 and 5.7-fold lower than for larvae 3 and 5 d in age, respectively (based on 95% confidence intervals). These data indicate that in cotton pest management systems, B. thuringiensis applications need to be directed at maximum oviposition and eclosion of H. zea neonates because control is very low against larger larvae.