Temporal and spatial regulation of cell division is critical for proper development of multicellular organisms. An important aspect of this regulation is cell-cycle arrest, which in many cell types is coupled with differentiated status. Here we report that the polar cells--a group of follicle cells differentiated early during Drosophila oogenesis--are arrested at G2 phase and can serve as a model cell type for investigation of developmental regulation of cell-cycle arrest. On examining the effects of String, a mitosis-promoting phosphatase Cdc25 homolog, and Notch signaling in polar cells, we found that misexpression of String can trigger mitosis in existing polar cells to induce extra polar cells. Normally, differentiation of the polar cells requires Notch signaling. We found that the Notch-induced extra polar cells arise through recruitment of the neighboring cells rather than promotion of proliferation, and they are also arrested at G2 phase. Notch signaling is probably involved in down-regulating String in polar cells, thus inducing the G2 cell-cycle arrest.