Insecticyanin was found to be synthesized in several isoelectric forms and stored in the pigment granules in the epidermis. Both major epidermal forms (INS-a, pl 5.5; INS-b, pl 5.7) were found in the cuticle, but only the most basic form, INS-b, was present in the hemolymph. In vitro the epidermis synthesized and secreted both forms into both the cuticle and the medium. Isolation of two cDNA clones for insecticyanin followed by hybridization to epidermal mRNA showed the presence of only one 1.1 kb mRNA, but transcription of the longer cDNA yielded a RNA which produced INS-a but no INS-b. Insecticyanin mRNA was present during the intermolt feeding stages of the 4th and 5th instars and absent during the larval molt and after the onset of metamorphosis. Exposure of either day 2 4th-instar or day 1 5th-instar larval epidermis to 20-hydroxyecdysone (20HE) in vitro caused a dose-dependent decline in this mRNA that was not prevented by simultaneous exposure to JH. When synthesis resumes just before ecdysis, INS-b appears before INS-a; then on the final day of feeding, synthesis of INS-a ceases before that of INS-b. Culture experiments showed that exposure of day 15th epidermis to a small pulse of 20HE followed by an ecdysteroid-free period was best to mimic the in vivo situation, indicating that the small rise of ecdysteroid in the absence of JH on day-2 was responsible for this differential cessation.