Signal perception and the integration of signals into networks that effect cellular changes is essential for all cells. The self-incompatibility (SI) response in field poppy pollen triggers a Ca²⁺-dependent signaling cascade that results in the inhibition of incompatible pollen. SI also stimulates dramatic alterations in the actin cytoskeleton. By measuring the amount of filamentous (F-) actin in pollen before and during the SI response, we demonstrate that SI stimulates a rapid and large reduction in F-actin level that is sustained for at least 1 h. This represents quantitative evidence for stimulus-mediated depolymerization of F-actin in plant cells by a defined biological stimulus. Surprisingly, there are remarkably few examples of sustained reductions in F-actin levels stimulated by a biologically relevant ligand. Actin depolymerization also was achieved in pollen by treatments that increase cytosolic free Ca²⁺ artificially, providing evidence that actin is a target for the Ca²⁺ signals triggered by the SI response. By determining the cellular concentrations and binding constants for native profilin from poppy pollen, we show that profilin has Ca²⁺-dependent monomeric actin-sequestering activity. Although profilin is likely to contribute to stimulus-mediated actin depolymerization, our data suggest a role for additional actin binding proteins. We propose that Ca ²⁺-mediated depolymerization of F-actin may be a mechanism whereby SI-induced tip growth inhibition is achieved.