Stomatal regulation is essential for the growth of land plants. Pairs of guard rolls that delineate the stomata perceive stimuli and respond to acquire the optimum aperture. The actin cytoskeleton participates in signaling pathways of the guard tell (Kim et al., 1995; Eun and Lee, 1997; Hwang et al.,1997). To identify the upstream molecules that regulate actin dynamics in plant cells, we immunoblotted proteins extracted from Heaves of Commelina communis L. with the RhoA antibody and identified one band of 26KD from the epidermis. Using immunofluorescence microscopy we examined the subcellular distribution of the immune-reactant(s) in guard cells. When stomata were open under light, the organization of the immune-reactant(s) resembled the radial arrangement of cortical actin filaments of guard tells. Double-labeling of the guard cells, using the RhoA and actin antibodies as primary antibodies, showed that the immune-reactant(s) of the RhoA antibody and actin filaments co-localized in the cortex of illuminated guard cells. However the pattern was not found in guard cells when stomata were closed under darkness or by ABA, conditions under which cortical actin proteins are disassembled in guard cells. From these observations, we can suggest the possible presence of a RhoA-like protein and its involvement in the organization of the actin cytoskeleton in guard cells.