A peptide from cadherin AgCad1 of Anopheles gambiae larvae was reported as a synergist of Bacillus thuringiensis subsp. israelensis Cry4Ba's toxicity to the Anopheles mosquito (G. Hua, R. Zhang, M. A. Abdullah, and M. J. Adang, Biochemistry 47:5101-5110, 2008). We report that CR11 to the membrane proximal extracellular domain (MPED) (CR11-MPED) and a longer peptide, CR9 to CR11 (CR9-11), from AgCad1 act as synergists of Cry4Ba's toxicity to Aedes aegypti larvae, but a Diabrotica virgifera virgifera cadherin-based synergist of Cry3 (Y. Park, M. A. F. Abdullah, M. D. Taylor, K. Rahman, and M. J. Adang, Appl. Environ. Microbiol. 75:3086-3092, 2009) did not affect Cry4Ba's toxicity. Peptides CR9-11 and CR11-MPED bound Cry4Ba with high affinity (13 nM and 23 nM, respectively) and inhibited Cry4Ba binding to the larval A. aegypti brush border membrane. The longer CR9-11 fragment was more potent than CR11-MPED in enhancing Cry4Ba against A. aegypti.