Specializations of extracellular structures opposite hemidesmosomes at the basal surface of epithelial cells of the fetal rat urogenital sinus are described. A moderately dense band or lamella ∼ 100 Å wide forms a cap about 50 Å from the plasma membrane. The normally electron-lucent inner zone between the plasma membrane and the basal lamina is increased in density and may be traversed by filaments or strands of material extending from the cell surface. The basal lamina is thickened and its density is increased locally opposite hemidesmosomes. The possible role of these specializations in adhesion between epithelial cells and the underlying extra-cellular structures is considered.