Ecdysteroids acting through multiple isoforms of the ecdysone receptor (EcR) initiate molting and metamorphosis of insects. Two isoforms of EcR, A (this paper) and B1 (Fujiwara et al., Insect Biochem. Mol. Biol. 25, 845-856, 1995), were isolated from the tobacco hornworm, Manduca sexta, and shown to be similar to the corresponding Drosophila EcR isoforms. The developmental profiles of both EcR-A and EcR-B1 (determined by both analysis of isoform-specific mRNAs and use of monoclonal antibodies that detect either EcR-B1 or all forms), however, were different in Manduca epidermis, which produces sequentially the larval, the pupal, and the adult cuticles. EcR-B1 predominated through the larval, pupal, and early developing adult stages with an upregulation early in each molt. By contrast, EcR-A was present only at the onset of new cuticle synthesis during the larval molt, but in the pupal and adult molts was upregulated slightly later than EcR-B1 during the commitment period and was present during the predifferentiative phase. Both isoforms appeared in the larval wing discs after pupal commitment and persisted through pupal differentiation. The mRNAs for both isoforms were directly induced in larval epidermis in vitro by 20-hydroxyecdysone, but EcR-B1 mRNA accumulated more rapidly, peaking at 3 hr. In the presence of a protein synthesis inhibitor, the accumulation of EcR-B1 mRNA was slower and its subsequent decline was prevented, but the accumulation of EcR-A mRNA was unaffected. Thus, in this polymorphic epidermis both isoforms appear in every molt, with EcR-B1 present during the commitment and predifferentiative phases and then at the onset of cuticle synthesis EcR-A prevails. Additionally, EcR-A is apparently associated with the switching and predifferentiative events necessary for a new synthetic program.