Summary. Individual bull spermatozoa were tracked over a distance of several millimetres, measuring as often as possible the time required to swim the length of the micrometer (310 μ). In oestrous mucus from the cow's uterine cervix, the rates of some spermatozoa were found to fluctuate widely, while others remained constant for long distances. Homogeneity of the medium was tested by comparing mean rates of groups of spermatozoa in different regions of the mucus. The material was not sufficiently heterogeneous to account for the results. The timing error could account for only a small part of the variation. Similar fluctuations in swimming rate were observed in saline and in the supernate obtained by spinning off spermatozoa from a 20,000/μ1 suspension in saline.