This ultrastructural study was undertaken to investigate the morphological changes which occur in the fast twitch gastrocnemius muscle of the reptile Iguana iguana after nerve section. It was found that initial degenerative alterations appeared in muscle fibers two weeks after denervation and progressed along the two months of the investigative period. They consisted of disorganization of contractile and sarcotubular elements and the appearance of autophagic vacuoles with mitochondrial debris. However, even two months after nerve section some myofibrils and mitochondria looked normal. Our results suggest that although the general course of denervation atrophy in iguana gastrocnemius is similar to that in other twitch muscles of vertebrates, the chronology of the process shows that iguana fast twitch skeletal muscles exhibit an intermediary position among the vertebrates in relation to their velocity of response to denervation.