Observations on the development of the parasite T. mactator Tosquinet revealed that the 1st instar larva did not develop to the 2nd instar until their host P. xuthus L. pupated. To learn the effect of the physiological condition of the host on the development of its parasite, the following experiments were carried out. The parasitized 5th instar larva was ligatured and the development of the parasite was examined. When the abdomen of the ligatured host pupated, the parasite developed to the 2nd instar, but did not if it remained as a larva. When the diapausing host pupa was exposed to a long day photoperiod after pupation, the parasite left the host, while it entered into diapause under a short day condition in the host pupa. The process to the pupation of the host is an important factor in the revival of molting of the 1st instar parasite, but the development of the parasite will be further influenced by other factors, possibly photoperiodic condition. none