Movement characteristics of 'vanguard' and 'following' human spermatozoa within cervical mucus were measured using high-speed cinemicrography. The data were applied to a mathematical model of the hydrodynamics of sperm-mucus interaction, using the methods of analysis of covariance and stepwise multiple regression. Vanguard spermatozoa swam more rapidly and more efficiently than did following spermatozoa, although the flagellar beat frequencies and amplitudes of the spermatozoa in the two populations did not differ. This distinction in sperm-mucus hydrodynamics could be due to reduced flagellar thrust and/or increased mucous resistance experienced by the following spermatozoa.