Summary. Movement characteristics of freely swimming spermatozoa were studied with high-speed cinemicrography. At 21°C, flagellar beat frequency was higher in midcycle human cervical mucus than in native semen or Tyrode's solution; the beat shape differed, possessing diminished amplitude and wavelength. Although the spermatozoa swam straighter in the mucus, the progressive swimming speeds did not differ in the three media. Swimming speed and beat frequency were linearly related in semen and in Tyrode, but in mucus the linearity was less certain. In midcycle cervical mucus at 37°C, beat frequencies and swimming speeds were greater than at 21°C, but the trajectories were equally straight, and the distances swum per beat (kinetic efficiencies) did not differ.