Twenty two mid-trimester human fetal tracheae were processed for light microscopy and scanning, transmission and freeze-fracture replication electron microscopy. The tracheal pseudostratified columnar epithelium was found to be composed of four cell types: ciliated in various stages of development, non-ciliated, basal and degenerating cells. The ratio of ciliated to non-ciliated cells was approximately one-to-one. The non-ciliated cells were of two morphological sub-types. Of these, the vast majority contained characteristic secretory granules, approximately 0.5 microns diameter, found in abundance within the apical region of the cell cytoplasm. The second sub-type of the non-ciliated cell, the well differentiated goblet cell, was observed very infrequently. The relatively large number of non-ciliated cells in the fetal tracheal epithelium suggests a transition stage to the adult where the ratio of ciliated to non-ciliated cells is near 5-to-1. The nature of the first type of the non-ciliated cells, which have not been studied in depth previously, remains obscure. Their probable secretory function is discussed and comparisons are made with previously reported cell types showing similar features, in fetal or adult material from the distal airways of animal and human tissues. Approximately 15 to 20% of the epithelial cells show signs of degeneration. The probable causes of this degenerative process are discussed. Other cell types such as brush cells, endocrine and wandering lymphocytes were not observed. Well-differentiated muco-serous glands were observed within the tela submucosa.