Studies were carried out, using transmission electron microscopy, of the cilia of the nephrons of rat and human kidneys. Cilia were observed in the parietal layer of Bowman's capsule, in the proximal tubule, the distal tubule, including the macula densa, and the collecting duct. They had a number of characteristic features, including the presence of a centriole adjacent to the basal body, long, slender cross-striated rootlets, and a typically organized basal body. The shaft of the cilia differed from the typical 9 + 2 pattern of organization. Near the base of the cilia the pattern was 9 + 0. In the middle portion, one or more of the peripheral doublets had been displaced centrally to give an 8 + 1 or a 7 + 2 pattern, while towards the tip the pattern became more irregular and the doublets were reduced to single microtubules. We have hypothesized that these cilia may be vestigial. They may, if motile, have some minor stirring function, or they may play a sensory role, as has been postulated for similarly structured cilia in other sites.