Author made a histologic study of the middle ear mucosa obtained from 35 Korean fetuses of gestational ages ranging from 16 to 40 weeks. These fetuses were products of artificial abortion and were proven to be normal by autopsy. Temporal bones were removed from the fetuses as soon as possible after expiration, and were fixed in 10% buffered formalin. After decalcification process with 4% nitric acid, the specimens were embedded in paraffin, and 10μm serial sections of horizontal plane were made. Staining was done with Hematoxylin and Eosin. Following results were obtained: 1) Lining epithelium in middle ear cavity ranged from squamous, low cuboidal, cuboidal, stratified cuboidal, stratified ciliated cuboidal, columnar, ciliated columnar, stratified ciliated columnar to pseudostratified ciliated columnar epitheliums by different areas. 2) Middle ear mucosa was a continuation of the respiratory epithelium in the nasopharynx which was preserved in the areas surrounding its nasopharyngeal orifice of Eustachian tube and the epithelium of the posterior part of the tympanum was a modified respiratory epithelium, which was developing to respiratory epithelium. 3) Eustachian tube orifice was covered mainly by pseudostratified ciliated columnar epithelium at 16 weeks gestational age. 4) In the hypotympanum, respiratory epithelium began to appear at 16 weeks gestational age, and by 31 to 35 weeks gestational period most of the hypotympanum was covered with respiratory epithelium. 5) Promontory and tympanic membrane were covered with squamous, low cuboidal and cuboidal epithelum but cuboidal epithelium was increased gradually with increasing gestational age from 16 gestational weeks on. 6) Epitympanum and attic were covered with squamous epithelium at 16 weeks gestational age, and by 36 to 40 weeks gestational period low cuboidal epithelium appeared frequently.