The crystallization phenomenon of human cervical mucus was studied with the scanning electron microscope. Changes in the conformation of the crystals, crystal conglomerates, and ferning patterns occurred almost daily. During the very early and late stages of the cycle two types of crystals could be found, one columnar and the other flat. At the other times, the columnar crystals were absent and the flat crystals changed their shape to cuboidal. During the follicular phase, these cuboidal crystals were initially located in small groups, then in parallel rows, and finally in three types of ferning patterns, one seemingly typical for the time of ovulation. These patterns reversed during the luteal phase of the cycle. The scanning electron microscope appears to be an extremely useful instrument for the evaluation of these phenomena.