Abstract Data on fast-growing cell populations in the mouse have suggested that the “doubling sequence” consisting of the S, G 2, and mitotic phases is of the constant duration of approximately 9 hr; differences in generation times seem to be largely due to different durations of the G 1 phase. This hypothesis was tested in a cell population which grows at a moderately slow rate, viz., the epidermis of the ear, which has a generation time of about 3 weeks. The durations of the various phases of proliferation and maturation were estimated by labeling with tritiated thymidine and obtaining the following data. fraction of cells labeled soon after labeling, fraction of mitoses labeled at various times after labeling, fraction of cells in the basal layer, and distribution of labeled cells. The following estimates were obtained: total generation time, about 24 days; S phase, about 30 hr; G 2 plus early prophase, about 6.5 hr; mid-portion of mitosis, about 3.8 hr; G 1 phase, more than 22 days; lifetime of mature cell, 20 1 2 days. Since the total doubling time is four times as long as in fast-growing cells, the hypothesis tested is disproved. On the other hand, the fraction of the generation time occupied by G 1 is much greater than in fast-growing tissues.