The normal secondary tympanic membrane in human fetuses was examined by transmission electron microscopy. The membranes in 5- to 9-month-old fetuses consist of the following three layers: (1) an outer squamous epithelial layer facing the middle ear, which is not formed until 4 months old; (2) a middle fibrous layer containing collagen, elastin, fibroblasts and fibrocytes, and (3) an inner layer of flat cells facing the scala tympani. Following the maturation of the fetus the epithelium is getting thinner and fibroblasts are reduced in number, but fibrocytes are increased and collagen and elastin grow gradually in density. The ultrastructure of the secondary tympanic membrane at 8 month is mature in type and shows the same characteristics as in the adult. This membrane has an important and complicated physiological function. The epithelium of the outer layer, with tight junctions and multiple desmosomes, provides a barrier to keep harmful substances out. The stability of the membrane provides protection against rupture, while the elasticity plays a role in the physiology of hearing as well.