The swallowtail butterfly, Papilio xuthus L., exhibits seasonal diphenism, i.e., spring and summer morphs, the development of which is determined by photoperiod and temperature imposed during the larval stage. Larvae reared under long day conditions at 25°C develop into summer morphs without pupal diapause, while larvae reared under short day conditions at 20°C develop into spring morphs after diapause development at 4°C for more than three months. To investigate the neuroendocrine mechanism regulating seasonal morph development, the activity of the summer-morph-producing hormone (SMPH) was tried to assayed by using male short day pupae of P. xuthus that accomplished diapause development at 4°C. The wing patterns of male butterflies shifted toward summer morphs when short day pupae were treated with the SMPH at an adequate stage. SMPH activity was detected in 2% NaCl extracts of the brain-subesophageal ganglion (Br-SG) complexes of P. xuthus long day pupae. Chilled male short day pupae responded to P.xuthus SMPH in a dose-dependent manner and lost the responsiveness within 2 days of transfer to room temperature (about 25°C). This is a report to show the existence of SMPH in butterfly species having pupal diapause.