In order to describe the development of the mucociliary system in the murine eustachian tube and middle ear, seventy six mice of ages ranging from gestational day 11 to postnatal day 21 were investigated with the light and electron microscopic observations. Development of the ciliated cells was seen concurrently in both the eustachian tube and middle ear on the 16th gestation day, one day earlier than the epithelial secretory cells appeared in both the eustachian tube and middle ear. The number of ciliated cells and secretory cells increased rapidly after birth. Tubal glands were well identified with evidence of secretory activity around the time of birth. Thus, the findings of this study suggest that the mucociliary defense system starts to develop during the fetal period and is well estabilished immediately after birth.