Recent scanning electron microscopic studies confirm the presence of solitary cilia on most epithelial cells along the mammalian nephron and collecting ducts. By transmission electron microscopy we have found that the axonemata of such cilia consist of a maximal number of 9 doublet and no singlet filaments. 10% of the cross-sectioned cilia contain 9 doublets arranged in peripheral ring (9 + 0 pattern). 30% of the cross-sections contain 8 or 7 doublets in peripheral ring and 1 or 2 doublets in the central region (8 + 1 and 7 + 2 patterns). Serial sections and goniometer tilt reveal the central doublets to originate as dislodged peripheral doublets. 60% of the sectioned cilia contain filament numbers between 8 and 4. In patterns of 5 and 4 filaments single microtubules predominate. The functional significance of these atypical cilia is discussed.