Ciliogenesis was investigated in the tracheal epithelium of human fetus at mid trimester of gestation (15~22 weeks), and the substructure of basal body was studied with serial, cross sections. The ciliogenic cells were long columnar cells with an electron -lucent cytoplasm, and contained rich free ribosomes and smooth endoplasmic reticulum. Apical cytoplasm of these cells contained various structures related to ciliogenesis including fibrous granules, procentrioles, centrioles and basal bodies. Basal bodies were located near apical plasma membrane and had basal foot and striated rootlets. In cross section, alar sheets appeared at transitional area between distal portion of basal body and axoneme, and basal foot at distal portion of basal body. Alar sheets arouse from each peripheral triplets of basal body and projected radially clockwise in apex to base view. Basal foot was a cone shaped structure with cross striation which base attached to two or three of the peripheral triplet sets and apex converged to basal foot cap. Three dimentional reconstruction by serial cross section of the basal body showed a structural relationship of alar sheets and basal feet with basal body. By immunohistochemistry, alpha -tubulin label was seen in both basal and surface ciliated cells, and gamma-tubulin label was seen in the apical region of surface cilated cells. These results indicate that ciliogenesis of tracheal epithelium of human fetus is performed mainly by acentriolar ciliogenesis, and suggest the ciliogenesis and ciliary movement at mid trimester of gestation are active.