Cited2 (CBP/p300 interacting transactivator with ED-rich tail 2) is required for embryonic development, coactivation of transcription factor AP-2, and inhibition of hypoxia-inducible factor 1 transactivation. Cited2 is induced by multiple growth factors and cytokines and oncogenically transforms cells. Here, we show that the proliferation of Cited2 ^−/− mouse embryonic fibroblasts ceases prematurely. This is associated with a reduction in growth fraction, senescent cellular morphology, and increased expression of the cell proliferation inhibitors p16^INK4a, p19^ARF, and p15^INK4b. Deletion of INK4a/ARF (encoding p16^INK4a and p19^ARF) completely rescued the defective proliferation of Cited2^−/− fibroblasts. However, the deletion of INK4a/ARF did not rescue the embryonic malformations observed in Cited2 ^−/− mice, indicating that INK4a/ARF-independent pathways are likely to be involved here. We found that Cited2 ^−/− fibroblasts had reduced expression of the polycomb-group genes Bmi1 and Mel18, which function as INK4a/ARF and Hox repressors. Complementation with CITED2-expressing retrovirus enhanced proliferation, induced Bmi1/Mel18 expression, and decreased INK4a/ARF expression. Bmi1- and Mel18-expressing retroviruses enhanced the proliferation of Cited2 ^−/− fibroblasts, indicating that they function downstream of Cited2. Our results provide genetic evidence that Cited2 controls the expression of INK4a/ARF and fibroblast proliferation, at least in part via the polycomb-group genes Bmi1 and Mel18.