After perfusion of the animals with buffered aldehyde solutions, spinal cords of normal dogs, cats and monkeys were prepared for electron microscopic examination by standard methods. Observations were limited to the upper cervical levels in all animals, except in two cats where thoracic and lumbar regions also were studied. Large mitochondria, up to 5.0 μ in diameter, are abundant in astrocytes that form a dorsolateral glial septum in dogs and cats. The septum is present but less well developed in monkeys and the accompanying large mitochondria are smaller and less numerous. These mitochondria have a moderately dense matrix and few or no cristae. They are accompanied by more numerous smaller mitochondria with similar matrix and prismatic cristae. The cristae are of constant size and shape in all three species but vary greatly in numbers and arrangement. It is suggested that local mechanical forces may be responsible for the configuration of these morphologically specialized mitochondria.