We have prepared several truncated versions of the Choristoneura fumiferana ecdysone receptor (CfEcR) cDNA to identify domains required for heterodimerization with C. fumiferana ultraspiracle (CfUSP). The CfEcR protein containing all six domains bound to hsp27 EcRE in the presence of CfUSP protein. The only regions that could be deleted from CfEcR without losing DNA binding activity were A/B and F domains as well as a 29 amino acid region located at the C-terminal end of the E domain. This suggests that the minimum region required for DNA binding is a 363 amino acid peptide spanning C, D, and part of E domains. The CfEcR protein containing all six domains bound ponasterone A, in the presence of CfUSP protein. Removal of the F domain and the C-terminal 12 amino acids in the E domain completely abolished ponasterone A binding. Removal of A/B and C domains did not affect ligand binding. However, deleting the D domain abolished ligand binding completely. The minimum region required for ligand binding is a 333 amino acid peptide spanning the D, E, and F domains. The A/B region is the only domain that can be deleted without affecting both DNA and ligand binding of CfEcR. These results suggest that CfEcR has two sub domains in the D and E domains that are necessary for heterodimerization with the CfUSP protein. Arch.