In the urodelan amphibian Pleurodeles waltlii, spontaneous external metamorphosis was correlated with an increase in the serum level of thyroxine (T4). Within the same period, a change occurred in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibres of larval type were gradually replaced by transitional fibres (type IIC), then by adult fibres of the types I, IIA, and IIB. Likewise, a myosin isoenzymic transition was observed. In larval animals, myosin electrophoresis revealed 3 bands corresponding with isoforms having identical heavy chains (MHC), but different light chains (MLC). In the course of metamorphosis, the 3 larval isomyosins were replaced by 3 isoforms having the adult type MHC and different motility. In a related neotenic species, Ambystoma mexicanum, no spontaneous anatomic metamorphosis occurred; at the time it should theoretically take place, the serum T4 level remained low. The ATPase profile was modified, but transitional fibres that replaced the initial larval types appeared to be persistent, and adult fiber types appeared only in a small amount. Myosin isoenzymic transition was also incomplete, larval isoforms were still distinguished in the neotenic adults. Similar persistence of larval characters was observed in adult Proteus anguinus, a perennibranch that never undergoes anatomical metamorphosis. Experimental hypothyroidian Pleurodeles waltlii displayed no external metamorphosis, only the larval fibre types and isomyosins were detected in those animals. External metamorphosis was induced in Ambystoma mexicanum by a triiodothyronine treatment. A complete myosin isoenzymic transition was observed in metamorphosed animals. These results tend to indicate that a moderate increase in the level of thyroid hormones is sufficient to determine the production of the adult type MHC molecules and the differentiation of the corresponding myofibrillar types in the skeletal dorsal muscle of amphibians, while a marked increase would be necessary for repressing the initial larval feature.