Using a PCR strategy, we have cloned the cDNA for prothoracicotropic hormone (PTTH) from the giant silkmoth, Antheraea pernyi. The A. pernyi PTTH cDNA encodes a preprohormone of 221 amino acids that is 51 and 71% identical at the amino acid level with Bombyx mori and Samia cynthia ricini PTTHs, respectively. Bacterially expressed, recombinant A. pernyi PTTH stimulates adult development when injected into debrained pupae. PTTH protein (ca. 30 kDa by Western blot) and mRNA (ca. 0.9 kb by Northern blot) are expressed in brain. Immunocytochemistry and in situ hybridization show that PTTH protein and mRNA are colocalized in L-NSC III from Day 4 of embryogenesis through adult life, with little variation in either protein or mRNA levels at the various ecdyses. A pair of cells expressing immunoreactivity for the circadian clock protein PER is located in the same region as PTTH-expressing L-NSC III in A. pernyi brain. However, double-label immunocytochemical studies show that PTTH and PER are located in different cells. The close anatomical location between PTTH- and PER-expressing cells suggests routes of communication between these two cell populations that may be important for the circadian control of PTTH release.