Mushroom bodies (MBs) are insect brain centers involved in sensory integration and memory formation. In social Hymenoptera, MBs are large and comprise larger number of Kenyon cells and have repeatedly been implied to underlie the social behaviors. In the present study, to facilitate our understanding of the neural basis of social behaviors, two complementary DNAs (cDNAs) encoding presumed ecdysone receptor isoforms (CjEcR-A and CjEcR-alpha) were identified in the developing brains of the carpenter ant Camponotus japonicus. Sequence comparison indicated that these CjEcR proteins had common DNA- and hormone-binding domains linked to different N-terminal regions. The alignment of the distinct regions with other insects EcRs indicated that CjEcR-A is the ant homologue of EcR-A, and CjEcR-alpha has a novel type of A/B region. Immunohistochemical analyses of the MBs of C. japonicus with the common region antibody demonstrated that these CjEcRs appear in all neuroblasts, neurons, and glia cells during neurogenesis, whereas expression is confined to the neurons, disappearing in the glia cells in newly emerged workers. Less expression was observed in the forager MBs. These findings suggest that CjEcRs are involved in maturation and development of ant MBs.