Insecticides belonging to the four major groups were bioassayed against Helicoverpa armigera Hubner collected from cotton in the Guntur region of Andhra Pradesh, India, during the 1989/90, 1990/91 and 1991/92 cropping seasons. High levels of resistance to all groups of insecticides were recorded. Tolerance to quinalphos had increased drastically in the 1991/92 season, while resistance to endosulfan, monocrotophos and carbaryl had increased moderately. The populations were also highly resistant to cypermethrin and deltamethrin and mildly resistant to fenvalerate. The resistance levels varied from season to season, thus reflecting the selection pressure created through insecticides.