It is well known that cervical mucus restricts penetration of morphologically abnormal human sperm, both in vitro and in vivo. However, the mechanisms of such restriction are not well understood. Using videomicrography to simultaneously analyze the motions and morphology of individual human sperm, we analyzed differential penetration of normal and abnormal sperm into fresh human cervical mucus. Abnormal sperm swam slower in mucus than the normal sperm, but their flagellar beat parameters were not commensurately different. Multivariate statistical analysis of the relationship between individual sperm velocity and flagellar beat parameters indicated that the heads of the abnormal sperm experienced greater resistance from the mucus than did normal heads. Differential mucus resistance, more than altered motile vigor, appears to be responsible for the restriction of abnormal sperm during migration through mucus.