Stomata, dynamic pores found on the surfaces of plant leaves, control water loss from the plant and regulate the uptake of CO(2) for photosynthesis. Stomatal aperture is controlled by the two guard cells that surround the stomatal pore. When the two guard cells are fully turgid, the pore gapes open, whereas turgor loss results in stomatal closure. In order to set the most appropriate stomatal aperture for the prevailing environmental conditions, guard cells respond to multiple internal and external signals. Although much is known about guard-cell signaling pathways, rather little is known about how changes in gene expression are involved in the control of stomatal aperture. We show here that AtMYB61 (At1g09540), a gene encoding a member of the Arabidopsis thaliana R2R3-MYB family of transcription factors, is specifically expressed in guard cells in a manner consistent with involvement in the control of stomatal aperture. Gain-of-function and loss-of-function mutant analyses reveal that AtMYB61 expression is both sufficient and necessary to bring about reductions in stomatal aperture with consequent effects on gas exchange. Taken together, our data provide evidence that AtMYB61 encodes the first transcription factor implicated in the closure of stomata.