Environmental factors shape the phenotypes of multicellular organisms. The production of stomata—the epidermal pores required for gas exchange in plants—is highly plastic, and provides a powerful platform to address environmental influence on cell differentiation [1-3]. Rising temperatures are already impacting plant growth, a trend expected to worsen in the near future . High temperature inhibits stomatal production but the underlying mechanism is not known . Here, we show that elevated temperature suppresses the expression of SPEECHLESS (SPCH), the bHLH transcription factor that serves as the master regulator of stomatal lineage initiation [6,7]. Our genetic and expression analyses indicate that the suppression of SPCH and stomatal production is mediated by the bHLH transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), a core component of high temperature signaling . Importantly, we demonstrate that upon exposure to high temperature, PIF4 accumulates in the stomatal precursors and binds to the promoter of SPCH. In addition, we find SPCH feeds back negatively to the PIF4 gene. We propose a model where the high temperature-activated PIF4 binds and represses SPCH expression to restrict stomatal production at high temperature. Our work identifies a molecular link connecting high temperature signaling and stomatal development, and reveals a direct mechanism by which production of a specific cell lineage can be controlled by a broadly-expressed environmental signaling factor. Overall design: Gene expression profiles following 12 hr Dex-induction of control and ML1p:SPCH1-4A-expressing Arabidopsis plants grown in liquid culture. Four replicates per line at 0 and 12 hr.