With the periodic acid-thiosemicarbazide-silver proteinate procedure for the detection of polysaccharides in thin sections, glycogen is localized in the cavities of centrioles and basal bodies, within the axoneme (and surrounding it), in mitochondria, and in the "packing" cytoplasm of the middle piece of spermatozoa of several invertebrate and vertebrate species. The cytochemical localization of glycogen is verified by extraction with alpha-amylase solution. These findings establish the existence of stored glycogen in sperm. The polysaccharide presumably serves as an endogenous source of energy in the absence of extracellular metabolites, under either aerobic or anaerobic conditions. Other hypotheses on the physiological significance of intracellular glycogen stores in sperm are discussed. Sperm that store glycogen contain some enzymes of glycogen metabolism. In the presence of glucose-1-phosphate, ATP, and Mg(++) ions, an amylophosphorylase catalyzes the in vivo synthesis of glycogen. The newly formed product resembles gamma-particles, and is digestible with alpha-amylase.