Movement characteristics of the seminal spermatozoa of 9 men of contemporary fertility were studied using time-exposure photomicrography. Five different ejaculates, collected over a 9-month period, were analysed for each man. The assessment of sperm motility focussed on classical parameters (% motile spermatozoa and the total swimming speed, considering all motile spermatozoa) as well as several newly defined movement characteristics (% progressive spermatozoa, progressive swimming speed, and % straight-swimming, rolling and yawing spermatozoa). These new measures distinguished weakly motile from vigorously motile spermatozoa, and also assessed geometrical attributes of the swimming trajectories of the vigorous spermatozoa. The % motility and the total and progressive swimming speeds did not differ significantly amongst the fertile men. The percentage of progressive spermatozoa differed significantly among the 9 men, but this was due to the outlying values of a single donor. Significant differences among men did occur in the percentages of straight-swimming, rolling and yawing spermatozoa. In addition, considerable variability was found among the different ejaculates of individual men.