AbstractGross anatomy, ultrastructure, innervation and ultrastructural alterations of the prothoracic gland (PTG) of cotton bollworm,Helicover pa armigera(Lepidoptera: Noctuidae) are illustrated for the last larval and early pupal stages as observed by light and electron microscopy. The T‐shaped, paired (PTGs) consist each of 76–116 cells which are classified morphologically as large and small gland cells. In addition, another kind of small (about 6μ in diameter) gland cell was found in the PTGs of last instar larvae. The PTGs are innervated by the branches of 3 nerves! and tracheae and tracheoles are abundantly distributed to these glands. PTGs disappeared completely by the third day after ecdysis to the pupal stage (at temperature 28 C with a photoperiod L15:D9). An intercellular channel system (ICS) is formed by numerous, deep invaginations of the plasma membrane of gland cells. This ICS gradually increases in depth and width and reaches maximum development around the time of the major ecdysteroid secretion peak during the last larval instar.Numerous multivesicular sacs (MVS) with their remnants and an extensive rough endoplasmic reticulum were observed within ICS and cytoplasm, respectively, on the fourth day of the last larval instar. At that time the matrix of mitochondria became much more electron lucent.Freeze‐fracture replicas of the glandular epithelium were made from last instar (4th day larvae. Dynamics of structure are related to data from others concerning secretory states of the prothoracic glands of this s