The movement characteristics of bull spermatozoa in fresh estrous bovine cervical mucus were studied using high-speed cinemicrography. Preparations consisted of capillary tubes of rectangular cross section (400 .mu.m deep) exposed to semen at one end. Spermatozoa were studied while swimming in the interior midplane of the tube and also in a plane just below the upper surface of the tube. In both these planes the initial or vanguard spermatozoa that arrived at a station 2 cm from the semen-mucus interface were studied in comparison with the subsequent or following spermatozoa that arrived 10 min later. The vanguard spermatozoa swam more rapidly than their followers, although the flagellar beat frequencies and shapes of these 2 groups did not significantly differ. This difference in propulsive efficiency appears to due an alteration in local mucus properties resulting from sustained permeation by spermatozoa. Spermatozoa swam more rapidly near the tube wall than in the midplane. Flagellar beat frequencies were lower near the wall and beat shapes were also qualitatively different. This latter distinction appears to result from altered mucus properties near the wall as well as the direct hydrodynamic effect of the wall itself.