In the developing vertebrate retina, progenitor cell proliferation must be precisely regulated to ensure appropriate formation of the mature tissue. Cyclin kinase inhibitors have been implicated as important regulators of proliferation during development by blocking the activity of cyclin-cyclin-dependent kinase complexes. We have found that the p27(Kip1) cyclin kinase inhibitor regulates progenitor cell proliferation throughout retinal histogenesis. p27(Kip1) is upregulated during the late G(2)/early G(1) phase of the cell cycle in retinal progenitor cells, where it interacts with the major retinal D-type cyclin-cyclin D1. Mice deficient for p27(Kip1) exhibited an increase in the proportion of mitotic cells throughout development as well as extensive apoptosis, particularly during the later stages of retinal histogenesis. Retroviral-mediated overexpression of p27(Kip1) in mitotic retinal progenitor cells led to premature cell cycle exit yet had no dramatic effects on Müller glial or bipolar cell fate specification as seen with the Xenopus cyclin kinase inhibitor, p27(Xic1). Consistent with the overexpression of p27(Kip1), mice lacking one or both alleles of p27(Kip1) maintained the same relative ratios of each major retinal cell type as their wild-type littermates. During the embryonic stages of development, when both p27(Kip1) and p57(Kip2) are expressed in retinal progenitor cells, they were found in distinct populations, demonstrating directly that different retinal progenitor cells are heterogeneous with respect to their expression of cell cycle regulators.