Abstract: Natural variability in susceptibility of field collected rice yellow stem borer (Scirpophaga incertulas) and rice leaffolder (Cnaphalocrocis medinalis) to Bacillus thuringiensis (Bt) δ-endotoxins was investigated to establish a geographic baseline for comparison of future population responses to the increased use of Bt based insect control products like Bt transgenic rice and commercial formulations. Populations of S. incertulas and C. medinalis were evaluated for their susceptibility to purified Bt Cry1Aa, Cry1Ab, Cry1Ac, Cry1C and Cry2A toxins. The range of LC50's among selected populations in response to variety of different Bt toxins were between 19-88 fold for S. incertulas and 2-20 fold for C. medinalis. The data provide a strong basis for monitoring changes in susceptibility of S. incertulas and C. medinalis populations to future use of Bt toxins in Bt rice program.