Bacillus thuringiensis (Bt) transgenic cotton, grown over about 3.8 million hectare in 2006 in India, is essentially a mosaic of different cry1 genes, predominantly based on Monsanto's 531 event carrying the cry1Ac gene, and to some extent JK AgriGenetics's event 1 with native modified cry1Ac, Nath Seeds's GFM event with fusion genes of cry1Ab and cry1Ac, and cry1Ab and cry1C in single-stacked hybrids, and the Monsanto's 15985 event with two cry1Ac and cry2Ab genes.  In view of different cry1Ac genes used for transformation, toxicity (96° h LC50) of some Cry1Ac proteins to the neonates of the cotton bollworm, Helicoverpa armigera, varied, with a range of 0.158?5.42° ìg/g for BGSC Cry1Ac (23 populations), 0.076?5.76° ìg/g for JK Cry1Ac (24 populations) and 0.0085?0.822° ìg/g for MVP Cry1Ac (21 populations).  There are significant differences in the toxicity of these Cry1Ac proteins; despite the use of genetically diverse insect populations, suggesting a need of evolving a consensus on Cry1Ac resistance monitoring of the larvae of H. armigera.