STOMATTA allow the diffusion of CO2 into the leaf for photosynthesis and the diffusion of H2O out of the leaf during transpiration1,2. This gaseous exchange is regulated by pairs of guard cells that surround each stomatal pore. During water stress the loss of water through transpiration is reduced in response to abscisic acid3, a naturally occurring plant growth regulator which is also present in certain mammals4, algae5 and fungi6, by the promotion of stomatal closure and inhibition of opening7. This involves alterations to guard cell turgor, causing the cells to shrink and thereby reducing the size of the stomatal pore. These changes are driven by cation and anion effluxes8. It has been proposed that an abscisic acid-dependent increase in the concentration of guard cell cytosolic free calcium triggers the intracellular machinery responsible for stomatal closure9(for a review, see ref. 10), but attempts to test this hypothesis by measuring [45Ca] fluxes have produced equivocal results11. Using the fluorescent calcium indicator fura-2, we report that abscisic acid induces a rapid increase in guard cell cytosolic free Ca2+ in Commelina communisL., and that this increase precedes stomatal closure. These results strongly support the suggestion that Ca2+ is an intracellular second messenger in this response.