Tetrahymena and Chlamydomonas cells treated with high (0.25-0.5%) concentrations of the detergent Nonidet P-40 in appropriate buffers retain the shape of the intact cells but are devoid of any ciliary activity unless supplied with MgATP. ATP causes them to swim actively, with beat parameters and swimming patterns indistinguishable from those of intact cells. Both types of detergent-extracted cells are completely devoid of ciliary membranes. The Tetrahymena preparations also lack all cellular membranes, whereas cellular membranes remain intact in the Chlamydomonas preparations. Experiments demonstrating the effects of ATP, ADP, vanadate, erythro-9-[3-2-(hydroxynonyl)]-adenine, and Ca++ are described to illustrate the use of these detergent-extracted cells in research on ciliary motility.