Dense aggregates of granular material, ranging from 400 to 1600 A in diameter, are present in the mitochondria of spermatozoa of Paracentrotus lividus. These intramitochondrial aggregates are intensely stained with lead citrate, are enhanced by lead stains after oxidation in H202 or periodic acid solutions, extracted with a-amylase or saliva, and are composed of 40 A subunits. Based on these cytochemical tests the dense intramitochondrial aggregates are thought to consist of glycogen particles. Glucose-6-phosphatase and adenosine triphosphatase activity are present in the middle piece of sea urchin spermatozoa. The enzyme reaction product lies next to the cristae and on the mitochondrial envelope. The presence of glucose6-phosphatase activity and glycogen in the mitochondria of spermatozoa of the sea urchin implies that glycolytic sequences are probably active in their metabolism.