cDNAs were isolated from
Manduca sexta
that encode two isoforms of an ultraspiracle (USP) homologue MsUSP‐1 and MsUSP‐2 with different N‐terminal A/B regions. The MsUSP‐1 cDNA predicts a protein with 97% and 45% amino acid identities in the DNA‐ and ligand‐binding domains respectively to the
Drosophila
USP and 89% overall identity with
Bombyx mori
CF1 (an USP homologue). Northern blot hybridizations with probes specific to MsUSP‐1 and MsUSP‐2 showed transcripts of an approximately equal size (4.5 kb), but with diverse developmental profiles in
Manduca
epidermis during the two final larval instars and the onset of the adult moult. The MsUSP‐1 mRNA was expressed during the intermoult periods, with higher levels around the time of the larval ecdyses and at the onset of wandering behaviour. In contrast, the MsUSP‐2 mRNA was up‐regulated at times of high ecdysteroid titre during the larval moults, when the MsUSP‐1 mRNA disappeared. Together, these conversely regulated isoform mRNAs contribute to the constitutive expression profile of total MsUSP mRNA.