An ultrastructural study on the prothoracic glands of fall-webworms (Hyphantria cunea Drury) was performed to reveal secretory activities of the ecdysone hormone in the five developmental stages of last-instar pupae, prepupae, pupae immediately after pupation, 3 -day-old pupae and diapausing pupae. The results obtained are as follows: The active prothoracic gland cells have well-developed smooth endoplasmic reticulum which produces the moderately electron-dense material which is presumed to be ecdysone hormone. The edysone, after being synthesized, is transported into vesicular structures formed at the marginal ends of the smooth tubular endoplasmic reticulum. Some vesicles produced by the separation of vesicular structures from the smooth tubular endoplasmic reticulum were incorporated into either other vesicles or smaller electron-lucent spheroids to form larger secretory electron-lucent spheroids. The spheroids containing the ecdysone are released into haemolymph. According to the ultrastructural characteristics associated with the secretory activities of the ecdysone, the prothoracic glands of the prepupae have high secretory activities, compared with the prothoracic glands of the last-instar larvae with the secretory activities which a few cells of their prothoracic glands have. But not only most of the prothoracic gland cells of the last-instar larve but also all the prothoracic gland cells of pupae just after pupation, 3-day-old pupae and diapausing pupae are inacitve in secretory activities. The inactive prothoracic gland cells have also more highly indented nuclei and more abundant glycogen granules to an extent of the stages of the pupae.