In previous work, the stomata of the maritime halophyte Aster tripolium L. were shown to close when NaCl concentrations rise in the vicinity of the guard cells. Further studies have now revealed important effects of calcium on the ionic responses of the stomata. When the guard cells were presented with KCl, Ca2+ suppressed opening in a manner similar to that which has become familiar in other species such as Commelina communis L. However, in the presence of NaCl, Ca2+ had the opposite effect, reducing the closing response to NaCl. This pattern of behaviour is discussed in relation to known salt effects on membranes, but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing response of the stomata to Na+ ions may make an important contribution to the salinity tolerance of this species. Here we report that increasing supplies of Ca2+ ions reduce the effect of salinity on stomatal conductance in the whole plant as well as in the isolated epidermis. This finding is consistent with the well established role of calcium in increasing resistance to salinity: in the presence of high calcium the plant can tolerate a greater salt intake, and hence there is a reduced need for transpiration to be restricted by partial stomatal closure.