The erector spinae muscles of the patas monkey, hamadryas baboon, and spider monkey were dissected and the origins and insertions were described in detail. In the two terrestrial monkeys, a well-developed thoracolumbar segment of the muscle was observed. This trait seems to correlate to the sagittal movement of this region required in the pronograde quadrupedal locomotion. On the other hand, the erector spinae muscle of the spider monkey is quantitatively not so well developed and the arrangement of the muscle bundles was simpler than in the terrestrial monkeys. The arboreal habit of the spider monkey seems not to demand powerful extension in the thoracolumbar junction, and spinal rotation seems to be more important.