1. The spermatozoa of the tick, Ornithodoros moubata, which until now have been reported to move without any apparent means of propulsion, have been examined under the light and electron microscopes.
2. The surface of the spermatozoon is covered with ridges, running parallel to the long axis of the body, 0.30 to 0.47 µ long and 500 to 1,000 Å wide.
3. In longitudinal section approximately every other ridge exhibits wave-like deformations.
4. On the basis of these sub-microscopic structures and the behaviour of polystyrene balls stuck to the surface of live spermatozoa, the hypothesis is put forward that tick spermatozoa move by propagating lateral bending waves along the ridges in an antero-posterior direction.
5. No ‘9 + 2’ or ‘9 + 9 + 2’ arrangement of fibrils has been observed anywhere in tick spermatozoa.
6. There is a striking though not complete resemblance between the sub-microscopic surface structure of tick spermatozoa and that of gregarines.