Initiation and development of the follicle population in mice were examined during embryonic and early postnatal life. The shape of the curve relating follicle density with age suggests that follicle intiation begins in the 14-day-old embryo and continues until about two days after birth. During the period of follicle intiation, groups of follicles are established in a regular and systematic way but at any particular age during this period, the stage of development of adjacent groups is not always synchronous so that when initiation ceases the composition and numerical size of different groups are highly variable. Although 75% of all groups contained three, four, or five follicles some were composed of only two and others as many as 11 folllices. In adult mice the most notable feature of the spatial distribution of follicles is often only the tendency for follicles to be arranged in rows. An examination of the follicle population about six days after birth showed that this feature is not incompatible with the earlier developmental grouping structure. A comparison of the proportions of different fiber types in the coat with the proportions of follicles occupying specific positions in the follicle groups was unable to establish any close relationship between follicle and fiber types. Evidently the type of fiber grown by a follicle is more dependent on the age when the follicle is initiated than on the position of the follicle in the follicle group.