Helicoverpa armigera is one of the major pests of cotton crops. Bt-cotton, (transgenic cotton) that contains cry genes from Bacillus thuringiensis, was introduced as a control measure against Heliothis virescens in the U.S.A. Cry genes encode for toxins that are toxic to lepidoteran pests. The effect of Bt-cotton (cv. NuOpal) on H. armigera population numbers and egg parasitism was investigated. Scouting was conducted once a week over two cotton-growing seasons at a cotton farm near Marble Hall. The number of damaged bolls, bollworm eggs and larvae were determined in non-Bt- (cv. Delta Opal) and Bt-cotton fields. No pesticides were used during the first season. A sprayed non-Bt cotton field was included in the second season\'s study. Pheromone traps baited with the sex pheromone of H. armigera were checked on a weekly basis to determine the number of adults in the fields under investigation. Bollworm eggs were collected during the second season to determine egg parasitism. The number of bollworm larvae and damaged cotton bolls were kept below threshold levels by the presence of the Bt-gene in the Bt-cotton fields and by insecticide applications in the non-Bt-cotton fields. Moth numbers, H. armigera oviposition and parasitoid emergence from bollworm eggs by Trichogrammatoidea lutea and Telenomus ullyetti were not influenced by the presence of the Bt-gene. Bt-cotton can therefore be useful in an integrated pest management programme under an irrigation system, because it resulted in a reduction in the number of insecticide applications per season and did not affect the biological control agents T. lutea and T. ullyetti. 