Technical Abstract: The sugarcane borer, Diatraea saccharalis, is a major target pest of transgenic corn expressing Bacillus thuringiensis (Bt) proteins in South America and the mid-southern region of the United States. Evolution of insecticide resistance in such target pests is a major threat to the durability of transgenic Bt crops. Understanding resistance mechanisms could greatly facilitate development of effective strategies for delaying or countering resistance. Cadherin- and alkaline phosphatase (ALP)-mediated resistance has been documented to be the two most common mechanisms of Bt resistance. In this study, neither the activity nor gene regulation of ALP was associated with Cry1Ab resistance in D. saccharalis with the ALP-mediated resistance mechanism. Total ALP enzymatic activity was similar between Cry1Ab-susceptible (Cry1Ab-SS) and -resistant (Cry1Ab-RR) strains of D. saccharalis. In agreement with this observation, expression levels of three ALP genes were also similar between Cry1Ab-SS and -RR, and their cDNA sequence did not differ between susceptible and resistant larvae. In contrast, altered expression of a midgut cadherin (DsCAD1) was associated with the Cry1Ab resistance. Whereas cDNA sequences of DsCAD1 were identical between the two strains, the transcript abundance of DsCAD1 was significantly lower in Cry1Ab-RR. To verify the involvement of DsCAD1 in Cry1Ab resistance, RNA interference (RNAi) was employed to knock-down DsCAD1 expression in the susceptible larvae. Down-regulation of DsCAD1 expression by RNAi was functionally correlated with a decrease in Cry1Ab susceptibility. These results suggest that down-regulation of DsCAD1, coupled with down-regulation of three aminopeptidases N genes shown in our previous study, are mechanisms of resistance to Cry1Ab in D. saccharalis.