Metachronal waves of ciliary activity on Opalina ranarum were instantaneously fixed, dehydrated by critical point drying, and examined with the scanning electron microscope. Photographs of waves on living Opalina were taken with Nomarski interference-contrast optics for comparison. Waves were also sectioned in three different planes for examination of ciliary cross-sections with the transmission electron microscope. This was done for metachronal waves travelling from anterior to posterior in some specimens and from right to left in others. Our object was to test the theory that the axis of the central pair of ciliary fibrils controls the direction of ciliary beating. The form of the wave and component parts of the beat are constant, no matter what is the direction of the wave across the animal, and comparison of the living with fixed material shows that the form is faithfully preserved. Moreover, in any region of a cilium, a line drawn through the central pair of fibrils is always at right angles to the direction of bending during both recovery and effective strokes, whatever the direction of the wave, but this observation alone does not show whether the plane of the central fibrils determines the direction of bending, or vice versa. Other considerations lead to the conclusion that the orientation of the central fibrils influences the direction of the effective stroke, but we cannot rule out the possibility that the direction of bending causes a passive twist of the central pair, especially during the recovery stroke.