Cytologic examination of oxazone-stained (nile blue) samples of amniotic fluid has become increasingly accepted as a reliable, inexpensive, and simple method of assessing fetal maturity. The present study focused on the ultrastructure of amniotic fluid cells in late human pregnancy. By means of light microscopy and scanning electron microscopy of the same cell in a series of cells, it was possible to observe the ultrastructural counterpart to the cell pattern indicative of fetal maturity. The type of cell indicating immaturity had a cytoarchitecture different from that of cells indicating maturity. The latter had an amorphous surface and coarse cytoplasmic strands; their oxazone-stainable, orange fat droplets were not found at electron microscopy. It is concluded that cells indicating maturity and those indicating immaturity differ in ultrastructure.