It has now been well established that during mammalian muscle development there is a sequential transition of the myosin isoforms, with the developmental isoforms being replaced just before or just after birth by the adult isozymes. In a previous study of human fetal muscle, we demonstrated the differentiation of two fiber populations as early as 15 weeks: one population of large diameter fibers containing predominantly slow myosin heavy and light chains, and another population which stained homogeneously for fetal myosin heavy chain and corresponded to histochemical type IIC fibers. We have carried out an immunocytochemical and biochemical study of human fetal quadriceps between 7 and 40 weeks. A chronology of the changes which occur in the expression of the myosin heavy and light chains is correlated with the results obtained by enzyme histochemistry. Evidence is also presented that in man excessive amounts of thyroid hormone act directly on the muscle, and result in a precocious accumulation of the adult myosin heavy chains and a precocious maturation of the muscle.