Single cilia (SC) in the articular cartilage of the cat are described with regard to structure, frequency and intracellular location. Comparison is made with their occurrence in the cartilage of other pecies or in other tissues; special consideration is given to their developmental stages. In the 1-year-old cat examined, about 20% of the cells showed SC with a predominantly intracellular course and some of them did not reach the extracellular space at all. Cross-sections of the proximal region of SC reveal the familiar 9+0 pattern; towards the distal end, the number of doublets and the diameter of the shaft are decreasing. It is concluded that most of the SC encountered are undergoing involution.