An artificial diet developed for the rice leaffolder, Cnaphalocrocis medinalis was used in a bioassay of endotoxin proteins from Bacillus thuringiensis (Bt) strains and plant-derived protease inhibitors against the leaffolder, C. medinalis, and the yellow stemborer, Scirpophaga incertnlas. Of the 14 samples of endotoxins derived from different strains of Bt (sub-species kurstaki and sphericus) tested, 5 samples — BT 29, BT 47, BT 61, BT 70 and HD-1— were effective against leaffolder larvae in preliminary tests. Subsequent tests indicated HD-1 to be the most effective against the leaffolder, followed by BT 61 and BT 70, with non-overlapping 95% fiducial limits of LC50 values (3.9,18.2 and 41.1 mg protein/kg diet, respectively). Mortality was observed after 48 h of release of neonate larvae. In contrast, the two protease inhibitors tested, namely soybean trypsin inhibitor (SBTI) and jackfruit seed trypsin inhibitor (JSTI) were less lethal to the two pests. The latter was more effective against the yellow stemborer at much lower concentrations.