Stomata are pores on the surfaces of leaves that regulate gas exchange between the plant interior and the atmosphere . Plants adapt to changing environmental conditions in the short term by adjusting the aperture of the stomatal pores, whereas longer-term changes are accomplished by altering the proportion of stomata that develop on the leaf surface [2, 3]. Although recent work has identified genes involved in the control of stomatal development , we know very little about how stomatal development is modulated by environmental signals, such as light. Here, we show that mature leaves of Arabidopsis grown at higher photon irradiances show significant increases in stomatal index (S.I.)  compared to those grown at lower photon irradiances. Light quantity-mediated changes in S.I. occur in red light, suggesting that phytochrome photoreceptors  are involved. By using a genetic approach, we demonstrate that this response is dominated by phytochrome B and also identify a role for the transcription factor, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) . In sum, we identify a photoreceptor and downstream signaling protein involved in light-mediated control of stomatal development, thereby establishing a tractable system for investigating how an environmental signal modulates stomatal development.