ABSTRACT.    Eukaryotic cilia and flagella are cytoskeletal organelles that are highly conserved from protists to mammals where they perform diverse motility and sensitivity functions. Defects in these complex organelles cause many genetic diseases, such as male infertility due to immotile sperm. Mammalian ciliated and flagellated cells are terminally differentiated and are difficult to cultivate, hampering their study. Trypanosoma brucei is a flagellated protozoan responsible for African sleeping sickness, and is turning out to be an excellent model for the study of cilia and flagella (Kohl et al, 2003, EMBO J. 22, 5336-46). Its genome is completely sequenced (Berriman et al, 2005, Science 309, 416-22). It can be easily cultivated in the laboratory and genetically manipulated. Potent and flexible tools for reverse genetics combined with tightly regulated tetracycline-inducible expression systems are available to monitor protein expression and assembly. Inducible expression of double-stranded RNA allowing for conditional RNA interference is used for functional studies. During its cell cycle, the trypanosome grows a new flagellum without disassembling the old one, thereby providing the possibility to compare a mature flagellum with a flagellum being assembled in the same cell (Bastin et al, 1999, Mol. Cell. Biol. 19, 8191-8200). Using this model we have been able to show that the flagellum is essential not only for cell motility, but is also involved in morphogenesis, cell division and organelle positioning (Absalon et al, PLoSONE, 2007, e437).