Two transgenic rice (Oryza satica L.) lines, KMD1 and KMD2, containing a synthetic cry1Ab gene from Bacillus thuringiensis Berliner, were evaluated for resistance to the rice leaffolder (RLF), Cnaphalocrocis medinalis (Guenee) under field conditions for 3 years in Zhejiang Province, China. Both KMD1 and KMD2 exhibited high and stable resistance against natural infestations by the RLF, and showed no symptoms of damaged leaves throughout the rice-growing season. In contrast, the untransformed parental control line (Xiushui 11) showed serious RLF damage symptoms not only in untreated plots, but also in plots treated once with chemical insecticides. These results demonstrate that both KMD1 and KMD2 have potential for protecting rice from the RLF damage.