Dormant cyst formation in ciliates such as Colpoda cucullus is a survival strategy for adaptation to terrestrial environments or unfavorable environments. The formation of dormant cysts involves drastic morphological and physiological changes: the structure of the vegetative cell is reorganized into a resting form that shows the loss of cell cycle activities and suppression of metabolic activity. In this review, we focus on the tolerance of Colpoda dormant cysts to gamma irradiation and cell repair after damage.