Regeneration is a process of rebuilding damaged or disrupted cells and tissues. The insect’s midgut epithelium becomes damaged by passage of the food mass and by secretory processes. The regeneration process occurs differently in two related species belonging to the primitive wingless insect group Zygentoma: Thermobia domestica (Packard) (Thysanura: Lepismatidae) and Lepisma saccharina L. (Thysanura: Lepismatidae). In T. domestica, the degenerated cells are replaced in a continuous manner by newly formed cells originating from regenerative cell groups. In L. saccharina, the midgut epithelium is totally removed and numerous regenerative cell groups form the new epithelium simultaneously in a cyclical manner. Regenerative cells, being responsible for all regenerative mechanisms, fulfill the role of primordial cells of the midgut epithelium. Here, I describe the process of degeneration and regeneration of the midgut epithelium in these two species at the transmission electron microscope level.