An attempt has been made to determine the sequence of cytological changes occurring during the uptake of water by ungerminated teliospores of Ustilago hordei. The first detectable changes are an increase in the amount of endoplasmic reticulum, the development of patches of "foamy cytoplasm," and a peculiar amoeboid activity of the nucleus. Shortly after, vacuoles begin to appear in the central regions of the protoplast. The mitochondria increase in size throughout the pregermination period but show no evidence of division until shortly before germination. Lipid does not seem to decrease substantially. Hypotheses are put forward to account for the origin of the endoplasmic reticulum and of "primary vacuoles."