There is now considerable evidence that the basis for ciliary and flagellar movement is an active sliding between peripheral doublet microtubules which, when resisted by structures within the axoneme, leads to axonemal bend formation. In contrast, relatively little is known about the control mechanisms which coordinate the interdoublet sliding and axonemal binding to produce the effective motion observed in various cilia and flagella. One component of the axoneme which may be involved in this control is the central pair of microtubules. To learn more about the action of the central pair, we have studied the tiny uniflagellate marine alga, Micromonas pusilla. The central tubules of the M. pusilla flagellum extend for several micrometres beyond the termination of the peripheral doublets, thus permitting direct observation of the central pair during flagellar movement. Our findings, reported here, indicate that in living M. pusilla the central pair of microtubules undergoes continuous rotation in one direction. This rotation provides the motive force for the cell.