The development of spontaneous locomotor behaviors was studied in the opossum Monodelphis domestica. The newborn opossum performs alternate, rhythmic movements with its forelimbs to crawl on the mother's belly where it attaches to a nipple, and its hindlimbs are little more than embryonic buds. The forelimbs retain the above movements for about 3 weeks, while the hindlimbs begin to move late in the second week. When detached from the nipple at 2-3 weeks, the pup can support its weight on the forelimbs and pivot around its hindquarter. Around the fourth week, the young can detach from the mother, its hindlimbs can support weight and linear locomotion appears, but the four limbs are not well coordinated. However, it can swim with coordinated movements of all limbs. Coordination when walking appears around the sixth week. During development, the duration of the step cycle decreases significantly. The durations of the stance and swing phases of the step cycle decrease in absolute terms, but swing increases as a percentage of the step cycle. The results are discussed in relation to the development of nervous and skeletomuscular components as well as sensorimotor reflexes.